The temperature of the sun is over 5000 degrees Fahrenheit at the surface, but it rises to perhaps more than 16 million degrees at the center．The sun is so much h
otter than die earth that matter can exist only as a gas, except at the core．In the core of the sun, the pressures are so great against the gases that, despite the high temperature, there may be a small solid core．However, no one really knows, since the center of the sun can never be directly observed．
Solar astronomers do know that the sun is divided into five layers or zones．Starting at the outside and going down into the sun, the zones are the corona, chromosphere, photosphere, convection zone and finally the core．The first three zones are regarded as the sun's atmosphere．
But since the sun has no solid surface, it is hard to tell where the atmosphere ends and the main
body of the sun begins．
The sun's outermost layer begins about 10000 miles above the visible surface and goes outward for mllions of miles．This is the only part of the sun that can be seen during an eclipse（日食） such as the one in February 1979．At any other time, the corona can be seen only when special instruments are used on cameras and telescopes to shut out the glare of the sun's rays．
The corona is a brilliant, pearly white, filmy light, about as bright as the full moon．Its beautiful rays are a sensational sight during an eclipse．The corona's rays flash out in a brilliant fan that has wispy spikelike（一束束穗状） rays near the sun's north and south poles．The corona is thickest at the sun's equator．
The corona rays are made up of gases streaming outward at tremendous speeds sad reaching a temperature of more than 2 million degrees Fahrenheit．The rays of gas thin out as they reach the space around the planets．By the time the sun's corona rays reach the Earth, they are weak and invisible．
小题1:Matter on the aim can exist only in fee form of gas because of the sun's ___．
| A．size | B．age | C．location | D．temperature |
小题2:All of the following are parts of the sun's atmosphere EXCEPT the ___．
| A．corona | B．chromosphere | C．photosphere | D．core |
小题3:According to the passage, as the corona ra5,'s reach -die plaasts, they become ___．
| A．hotter | B．clearer | C．thinner | D．stronger |
小题4:Which of 'die following do the paragraphs following the passage most likely discuss?
| A．The remaining layers of the sun． |
| B．The evolution of the sun to its present form． |
| C．The eclipse of February 1979． |
| D．All the above． |
affec
ted only the area immediately around large c
ities with factories and/or heavy automobile traffic. Today, we know that although these are the areas with the worst air pollution, the problem is literally worldwide. On several occasions over the past decade, a heavy cloud of air pollution has covered the entire eastern half of the United States and led to health warnings even in rural areas away from any major concentration of manufacturing and automobile traffic. In fact, the very climate of the entire earth may be affected by air pollution. Some scientists feel that the increasing concentration of carbon dioxide in the air resulting from the burning of fossil fuels(coal and oil) creating a greenhouse effect-holding in heat reflected from the earth and raising the world’s average temperature. If this view is correct and the world’s temperature is raised only a few degrees, much of the polar ice cap will melt and cities such as New York, Boston, Miami, and New Orleans will be under water.
United States
most brought worldwide effect 
