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.

　　63.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.

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

　　?A. the y circle around on their bikes

　　B. they use microcomputers without a stop

　　C. they exercise in one place for a long time

　　D. they watch a movie while pedaling

　　65.Some astronauts feel sick on the station during the first few days because _____.

　　A. their senses stop working

　　B. they have to stand up straight

　　C. they float out of their seats unexpectedly

　　D. whether they are able to go back to the station

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