仔细阅读下列短文, 根据题目, 选出最佳选项. We've talked about snails(蜗牛)and their slow move. But much of the time snails don't move at all. They're in their shells(壳)——sleeping. Hot sun will dry out a snail's body. So at the least sign of hot sun, a snail draws its body into its shell and closes the opening with a thin cover. Then it goes to sleep. A snail will die in a heavy rain. So whenever it rains, it goes inside its shell house and goes to sleep. A snail can sleep for as long as it needs to. It can take a short sleep. Or it can sleep for days at a time. And it spends all the winter months in its shell, asleep. In spring the snail wakes up. Its body, about three inches long, comes out from the shell. When hungry, the snail looks for food. It can't see very well. Its eyes, at the ends of the top feelers(触角)are very weak. But its sense of smell(味觉)is very strong. It helps the snail to the new greens. Then the snail's little mouth goes to work. A snail's mouth is no bigger than the point of a pin(针). Yet it has 256,000 teeth! The teeth are so small that you can't see them. But they do their work. If you put a snail in a hard paper box, it will eat its way out! And if a snail wears out its teeth, it will grow new ones. Mostly, a snail looks for food at night. But on cloudy days it eats in the daytime. It eats all day long. A snail can go on eating for hours and never feel full.

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阅读理解

　　At the moment，it may be difficult to imagine，but many people believe that，by the year 2100，we will live on the planet(行星) Mars.Our own planet，Earth，is becoming more and more crowded and polluted.We should build a better world on Mars.

　　First of all，transport should be better than they are today.Now it is difficult for spacecraft(宇宙飞船) to carry too large number of people to Mars梚t takes months.By 2100，travel to Mars will take a very short time in space shuttles that travel at half the speed(速度) of light.

　　Humans need water，air and food to survive.Scientists should be able to develop the plants that can grow on Mars.These plants can provide the air and food that we need.However，the scientists are not sure whether these plants can provide water.

　　Gravity(重力) will be the problem for the early settlers on Mars.The gravity on Mars will be about only three-eighth of the gravity on Earth.It is very dangerous for us because we can easily jump too high and float(漂浮) away into space.To prevent this，we have to wear special boots to make ourselves heavier.These boots will probably very popular and there will be many different designs for settlers to choose from.

　　Life on Mars will be better than life on Earth in many ways.We will have more space.Living in a dome with 10 bedrooms is highly possible.Many people thought that robots will do most of our work and we will have more time for our

hobbies.There will be no schools on Mars.Students will have a computer at home connecting to the inter-planet computer network.They can study，do homework and take exams in online schools.

　　However，in some ways，life on Mars may be worse than life on Earth.For example，food will be not the same as they are today，it may be in the form of pills(药片).

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