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 your’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 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 and Space Administration (NASA) puts it right on the workday schedule. The focus on fitness is as much about science and the future as it is about keeping any individual astronauts return home, and, more importantly, how to maintain strength and fitness for the two and a half years or more that it would take to make a round-trip to Mars.

　　26.What is the major challenge to astronauts when they sleep in space?

　　A. Deciding on a proper sleep position

　　B. Choosing a comfortable sleeping bag

　　C. Seeking a way to fall asleep quickly

　　D. Finding a right time to go to sleep.

　　27.The astronauts will suffer from a carbon-dioxide headache when _____.

　　 A. the y circle around on their bikes

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