The world is not hungry，but it is thirsty. It seems strange that nearly 3/4 of the earth is covered with water while we say we are short of (短缺) water. Why? Because about 97% of water on the earth is sea water which we can't drink or use for watering plants directly. Man can only drink and use the 3%—— the water that comes from rivers and lakes. And we can't even use all of that， because some of it has been polluted.

     Now more water is needed. The problem is: Can we avoid a serious water shortage (短缺) later on?

     First we should all learn how to save water. Secondly，we should find out the ways to reuse it. Scientists have always been making studies in the field. Today，in most large cities water is used only once and then runs to the sea or rivers. But it can be used again. Even if every large city reused its water，still there would not be enough. What could people turn to next?

     The sea seems to have the best answer. There is a lot of water in the sea. All that needs to be done is to get the salt out of the sea water. This is expensive， but it's already used in many parts of the world. Scientists are trying to find a cheaper way of doing it. So you see，if we can find a way out，we'll be in no danger of drying up.

根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。

     We often read the Air Quality Index （空气质量指标） or AQI reported in the newspaper. The AQI helps us understand what the air quality around us means to our health. The AQI uses colors, numbers and words to tell us about the air. Let's see what these colors and numbers mean.

    Mr Ellis is very old. He has seen many changes in his hometown.

He knows that things are different now. But he never forgets the old days. He likes to talk about them. He often talks about his favorite birds , the robins (一种小鸟—鸫).“When I was a boy, these houses were not here. There were wide fields and tall trees. Every spring the robins would come. Thousands of them would fly up to the north from the south. They spent the winter in the south. Some would build their nests(巢) in the trees near our houses. Then people started to cut down trees, and built more houses. The robins stopped coming. They couldn't build their nests near our houses because there were no trees.”

    “Now there are too many houses and too many roads. There are no places for the robins. They don't come any more.”

    In the universe, as we all know, there is the sun, the moon, the earth and a good many stars. Through our geography lessons, we know the earth goes around the sun, and the moon goes around the earth. We have day and night because the earth keeps turning all the time. When our part of the earth turns to the sun, it is day. When our part of the earth turns away from the sun, it is night.

Just because the moon is closer to the earth than the sun, it looks much bigger than the sun. Big things will look smaller when they are further, and small things also look bigger when they are nearer.

    The sun is big enough to give out very strong light. The moon can't give any light at all, but it looks quite bright, too. Why? In fact, the light from the moon comes from the sun. The moon looks much bigger and brighter than the stars. But actually (事实上) the stars are much bigger and brighter than the moon. They look smaller than the moon also because they are further away from the earth.

    There are many other planets in the universe. But of all the planets, only on the earth there are living things and people can only live on the earth, too. But at 9:00 a.m. on October 15th，2003, Chinese pilot Yang Liwei was sent up to space. He stayed there for 21 hours. It was the first time for Chinese to get into space.