What is the sky？ Where is it？ How bright is it？ What lies above it？ These questions are difficult to answer, aren’t they？

　　Is the sky blue？ The sky has no color. We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can’t fly very high because the air gets thinner when they go up higher. If we go up about 1,200 kilometers from the earth, we’ll find no air there.

　　When we talk about the sky, we usually mean the atmosphere around the earth. It is like a very thick “cover” over the earth and keeps it warm. Without the “cover”, the earth would be a very cold place and nothing could live on it.

　　Looking up above us, you will see the blue sky with white clouds. It is usually light blue, but the sky would be dark blue if you go to the top of the highest mountains. And you will see fewer clouds between you and the sun.

　　And if you sit in a spaceship high above the earth, there is no air at all. The sky would be so dark that its color would be black.

　　Sometimes the sky doesn’t look blue. When the sun rises and falls, you will see red, orange, and other colors in the sky. These are the most beautiful moments of the day.

　　Now you may understand that we see colors because the air scatters (散射) the sunlight. Light travels through the air. Without air, there would be no light and no colors.

1.Planes can’t fly very high because ____________.

A．there is air around the world

B．the air gets thinner when they go up higher

C．it’s warmer in the sky

D．the air gets thicker when they go up higher

2.We can see colors of the sky because ____________.

A．the sun rises and falls every day

B．the sky has lots of colors

C．the air scatters the sunlight

D．the sunlight has many colors

3.Which of the following is TRUE？

A．There is little air without light and colors in the sky.

B．The sky will look red, orange and other colors at noon.

C．The sky has only one color when the sun rises and falls.

D．The sky is dark blue if you go to the top of the highest mountains.

Jeff Keith has only one leg. When he was only twelve years old, Jeff had cancer. Doctors had to　16　 off most of his right leg.
　17　 Jeff puts on an artificial leg(假肢). The leg is plastic. With the plastic leg, Jeff can ski, ride a bicycle, swim, and play soccer. He can also run.
Jeff made a plan with his friends who had plastic legs, 　18　. They decided to 　19　 across America. They all wore special T-shirts.　20　 it was “Run, Jeff, Run, Jeff Keith’s Run Across America”.
Jeff Keith ran across the United States from the east to the west 　21　 he was twenty-two years old. He started running in Boston(波士顿). Seven　22　 later, he stopped　23　 in Los Angeles（洛杉矶）. He ran 3,200 miles (1 mile =" 1.6093km)." Jeff wore out thirty-six pairs　24　 running shoes and five plastic legs. Jeff　25　 in cities on the way to Los Angeles. In every city people gave Jeff money. The money was not for Jeff,　26　 for the American Cancer Society. The Society used the money to help people know 　27　 about cancer.
On the way to Los Angeles, Jeff talked to people about　28　. Jeff is disabled, but he can do many things. He finished college and is studying to be a lawyer(律师). Jeff says, “People can do　29　 they want to do. I want people to know that. I ran　30 for disabled people. I ran for everybody.”