1. Why does the author mention the questions in Paragraph 1?

　　A. To express his doubts.

　　B. To compare different ideas.

　　C. To introduce points for discussion.

　　D. To describe the conditions on Earth.

　　2. The underlined word "spin­offs" in Paragraph 4 probably refers to "________".

　　A. survival chances B. potential resources

　　C. unexpected benefits D. physical possessions

　　3. What makes it possible for humans to live on other planets?

　　A. Our genetic makeup.

　　B. Resources on Earth.

　　C. The adaptive ability of humans.

　　D. By­products in space exploration.

　　4. Which of the statements can best sum up the passage?

　　A. Space exploration has created many wonders.

　　B. Space exploration provided the best value for money.

　　C. Space exploration can benefit science and technology.

　　D. Space exploration may help us avoid potential problems on Earth.

　　B

　　The oddness of life in space never quite goes away. Here are some examples.

　　First consider something as simple as sleep. Its position presents its own challenges. The main question is whether you want your arms inside or outside the sleeping bag. If you leave your arms out, they float free in zero gravity, often giving a sleeping astronaut the look of a funny ballet (芭蕾) dancer. "I'm an inside guy，" Mike Hopkins says, who returned from a six­month tour on the International Space Station. "I like to be wrapped up."

　　On the station, the ordinary becomes strange. The exercise bike for the American astronauts has no handlebars. It also has no seat. With no gravity, it's just as easy to pedal violently. You can watch a movie while you pedal by floating a microcomputer anywhere you want. But station residents have to be careful about staying in one place too long. Without gravity to help circulate air, the carbon­dioxide you exhale (呼气) has a tendency to form an invisible (隐形的) cloud around your head. You can end up with what astronauts call a carbon­dioxide headache.

　　Leroy Chiao, 54, an American retired astronaut after four flights, describes what happens even before you float out of your seat. "Your inner ear thinks you're falling. Meanwhile your eyes are telling you you're standing straight. That can be annoying - that's why some people feel sick." Within a couple of days - truly terrible days for some - astronauts' brains learn to ignore the panicky signals from the inner ear, and space sickness disappears.

Space travel can be so delightful but at the same time invisibly dangerous. For instance, astronauts lose bone mass. That's why exercise is considered so vital that National Aeronautics