Throughout the past few centuries there have been recorded reports of space vehicles which have visited our Earth from another planet.  In more recent years, these reports of Unidentified Flying Objects(UFOs) have attracted a great deal of public interest.

    The reports of UFOs seem to increase over periods of a year or two, and then die down again. Many of the people who claim to have seen them are skilled observers of air, such as airline pilots.

    The number of reports runs into thousands, and they come from many parts of the world. There are also a number of supposedly authentic(可信的) photographs of these spacecraft which have earned them elves the nickname of "flying saucers"(飞碟) because of their fault, dish-like shape.

    So persistent have been these "sightings" that during the 1960s, the USA set up the National Investigation Committee on Aerial Phenomena (国家天象调查委员会) to investigate them. Nearly 11000 reports were analyzed (分析) and after eliminating (排除) sightings which could be explained away as normal phenomena, such as meteorites（损星）, high altitude(高空) weather balloons, and even satellites re-entering the atmosphere, they came to the conclusion that UFOs were genuine. They could not, however, explain what they were or from where they came.

    The United States Air Force, on the other hand, also carried out an evaluation(估计) of UFO sightings, and concluded that all but a very few of the thousands of sightings could be logically (合乎逻辑的)explained away, or traced to (追索为) some natural source, and the few unexplained ones were only unexplained because the evidence was insufficient(不足以) to draw any definite conclusion.

    Well, are UFOs spaceships from a distance planet, and are they under the control of extra-terrestrial things who are keeping close watch over our Earth? It can only be said that the case for UFOs remains as yet unproved. One thing is certain, man is as curious and determined to find the answer to the problem of UFOs as he is to unravel all the other unsolved mysteries of our Universe.

1．Why have people become more and interested in UFOs?

A．Because many of the people who claim(声称) to have seen them are skilled observers of the air.

B．Because there are some supposedly authentic photographs of UFOs.

C．Because there are continuing reports of UFOs.

D．All of the above.

2．According to the investigation by the U.S., UFOs________.

A．can not be explained away

B．really exist

C．can be explained away as normal phenomena

D．are from an unknown planet

3．Among the thousands of UFO sightings, the United States Air Force can logically explain away________.

A．a very few of them

B．all of them

C．almost all of them

D．many of them

4．The phrase “extra-terrestrial beings” means________.

A．spacemen

B．human beings

C．astronauts

D．airline pilots

    The reports of UFOs seem to increase over periods of a year or two, and then die down again. Many of the people who claim to have seen them are skilled observers of air, such as airline pilots.

    The number of reports runs into thousands, and they come from many parts of the world. There are also a number of supposedly authentic(可信的) photographs of these spacecraft which have earned them elves the nickname of "flying saucers"(飞碟) because of their fault, dish-like shape.

    So persistent have been these "sightings" that during the 1960s, the USA set up the National Investigation Committee on Aerial Phenomena (国家天象调查委员会) to investigate them. Nearly 11000 reports were analyzed (分析) and after eliminating (排除) sightings which could be explained away as normal phenomena, such as meteorites（损星）, high altitude(高空) weather balloons, and even satellites re-entering the atmosphere, they came to the conclusion that UFOs were genuine. They could not, however, explain what they were or from where they came.

    The United States Air Force, on the other hand, also carried out an evaluation(估计) of UFO sightings, and concluded that all but a very few of the thousands of sightings could be logically (合乎逻辑的)explained away, or traced to (追索为) some natural source, and the few unexplained ones were only unexplained because the evidence was insufficient(不足以) to draw any definite conclusion.

    Well, are UFOs spaceships from a distance planet, and are they under the control of extra-terrestrial things who are keeping close watch over our Earth? It can only be said that the case for UFOs remains as yet unproved. One thing is certain, man is as curious and determined to find the answer to the problem of UFOs as he is to unravel all the other unsolved mysteries of our Universe.

1．Why have people become more and interested in UFOs?

A．Because many of the people who claim(声称) to have seen them are skilled observers of the air.

B．Because there are some supposedly authentic photographs of UFOs.

C．Because there are continuing reports of UFOs.

D．All of the above.

2．According to the investigation by the U.S., UFOs________.

A．can not be explained away

B．really exist

C．can be explained away as normal phenomena

D．are from an unknown planet

3．Among the thousands of UFO sightings, the United States Air Force can logically explain away___

_____.

A．a very few of them

B．all of them

C．almost all of them

D．many of them

4．The phrase “extra-terrestrial beings” means________.

A．spacemen

B．human beings

C．astronauts

D．airline pilots

E
Could we "terraform" Mars —that is, change its frozen, thin-aired surface into something more friendly and Earthlike? Should we? The first question has a clear answer: Yes, we probably could．Spacecraft, including the ones now exploring Mars, have found evidence that it was warm in its youth, with rivers flowing into vast seas．And right here on Earth, we've learned how to warm a planet: just add greenhouse gases to its atmosphere．Much of the CO₂ that once warmed Mars is probably still there, in frozen dirt and polar ice caps, and so is the water．
Most of the work in terraforming, says NASA scientist Chris Mackay, would be done by life itself． "You don't build Mars," Mackay says．"You just warm it up and throw some seeds．" Powerful greenhouse gases could be produced from elements in dirt and air on Mars and blown into the atmosphere; by warming the planet, they would release the frozen CO₂, which would quicken the warming and increase atmospheric pressure to the point where liquid water could flow．Meanwhile, says botanist James Graham of the University of Wisconsin, human settlers could seed the red rock with a succession （系列） of ecosystems ?first bacteria and lichens （地衣）, which survive in Antarctica, later mosses （苔藓）, and after one thousand years or so, redwoods．Getting breathable oxygen levels out of those forests, though, could take thousands of years．
However, Mars is in no immediate danger．Some space scientists recently recommended going to the moon or an asteroid （小行星） first, and pointed out the space agency lacks the funds to go anywhere．It didn't estimate the cost of gardening a dead planet．
72．What is the most essential to make Mars fit for living?
A．Turn ice into flowing water．          B．Clean the dirty atmosphere．
C．Make the atmosphere more suitable．    D．Go to the moon or an asteroid first．
73．What does the author think of the idea ofterraforming Mars?
A．We could do it．                            B．We couldn't do it．
C．We should do it．                           D．We shouldn't do it．
74．In the botanist's opinion, man needs to __________.
A．enable Mars to clean its atmosphere
B．build a greenhouse on Mars
C．send some people to settle on Mars
D．cultivate ecosystems from lower to higher life form
75．What is the best title for the passage?
A．Making Mars the New Earth．           B．A Good Way to Change the Universe
C．Humans' Great Power．                     D．Terrible Science and Technology．

E

       Could we "terraform" Mars —that is, change its frozen, thin-aired surface into something more friendly and Earthlike? Should we? The first question has a clear answer: Yes, we probably could．Spacecraft, including the ones now exploring Mars, have found evidence that it was warm in its youth, with rivers flowing into vast seas．And right here on Earth, we've learned how to warm a planet: just add greenhouse gases to its atmosphere．Much of the CO₂ that once warmed Mars is probably still there, in frozen dirt and polar ice caps, and so is the water．

       Most of the work in terraforming, says NASA scientist Chris Mackay, would be done by life itself． "You don't build Mars," Mackay says．"You just warm it up and throw some seeds．" Powerful greenhouse gases could be produced from elements in dirt and air on Mars and blown into the atmosphere; by warming the planet, they would release the frozen CO₂, which would quicken the warming and increase atmospheric pressure to the point where liquid water could flow．Meanwhile, says botanist James Graham of the University of Wisconsin, human settlers could seed the red rock with a succession （系列） of ecosystems ?first bacteria and lichens （地衣）, which survive in Antarctica, later mosses （苔藓）, and after one thousand years or so, redwoods．Getting breathable oxygen levels out of those forests, though, could take thousands of years．

       However, Mars is in no immediate danger．Some space scientists recently recommended going to the moon or an asteroid （小行星） first, and pointed out the space agency lacks the funds to go anywhere．It didn't estimate the cost of gardening a dead planet．

72．What is the most essential to make Mars fit for living?

       A．Turn ice into flowing water．           B．Clean the dirty atmosphere．

       C．Make the atmosphere more suitable．    D．Go to the moon or an asteroid first．

73．What does the author think of the idea ofterraforming Mars?

       A．We could do it．        B．We couldn't do it．

       C．We should do it．       D．We shouldn't do it．

74．In the botanist's opinion, man needs to __________.

       A．enable Mars to clean its atmosphere

       B．build a greenhouse on Mars

       C．send some people to settle on Mars

       D．cultivate ecosystems from lower to higher life form

75．What is the best title for the passage?

       A．Making Mars the New Earth．     B．A Good Way to Change the Universe

       C．Humans' Great Power．        D．Terrible Science and Technology．