题干

中新网2010年4月23日报道,美国无人驾驶空天飞机X-37B于北京时间4月23日发射升空.如图所示,空天飞机能在离地面6万米的大气层内以3万公里的时速飞行;如果再用火箭发动机加速,空天飞机就会冲出大气层,像航天飞机一样,直接进入地球轨道,做匀速圆周运动.返回大气层后,它又能像普通飞机一样在机场着陆,成为自由往返天地间的输送工具.关于空天飞机,下列说法正确的是 (    )

A:它从地面发射加速升空时,机舱内的物体处于失重状态

B:它在6万米的大气层内飞行时,只受地球的引力

C:它在做匀速圆周运动时,所受地球的引力做正功

D:它从地球轨道返回地面,必须先减速

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答案(点此获取答案解析)

D

同类题2

阅读理解。

阅读短文,从每题所给的四个选项(A、B、C和D)中,选出最佳选项。

    Camping wild is a wonderful way to experience the natural world and, at its best, it makes little environmental influence. But with increasing numbers of people wanting to escape into the wilderness, it is becoming more and more important to camp unobtrusively(不引人注意地)and leave no mark.

    Wild camping is not permitted in many places, particularly in crowded lowland Britain. Wherever you are, find out about organizations responsible for managing wild spaces, and contact them to find out their policy on camping and shelter building. For example, it is fine to camp wild in remote parts of Scotland, but in England you must ask the landowner's permission, except in natural parks.

    Camping is about having relaxation, sleeping outdoors, experiencing bad weather, and making do without modern conveniences. A busy, fully-equipped campsite(野营地)seems to go against this, so seek out smaller, more remote places with easy access to open spaces and perhaps beaches. Better still, find a campsite with no road access: walking in makes a real adventure.

    Finding the right spot to camp is the first step to guaranteeing a good night's sleep. Choose a campsite with privacy and minimum(最小化的) influence on others and the environment. Try to use an area where people have obviously camped before rather than creating a new spot. When camping in woodland, avoid standing dead trees, which may fall on a windy night. Avoid animal runs and caves, and possible homes of biting insects. Make sure you have most protection on the windward side. If you make a fire, do so downwind of your shelter. Always consider what influence you might have on the natural world. Avoid damaging plants. A good campsite is found, not made—changing it should be unnecessary.

同类题3

   The teacher stood before his class and was going to hand out the final exam papers. “I know how 1you have all worked to prepare for this test,” he said. “And because I know you can do it well, I am willing to offer a B to anyone who would 2 not to take the test.
Many students jumped up to 3 the teacher and left the classroom. The teacher looked at the 4 students and said, “Does anyone else want to get a B? This is your last 5.” Two more students decided to go.
   Seven students remained. The teacher then handed out the 6. There were only three sentences typed on the paper: Congratulations! You have just 7 an A in this class. Keep believing in yourself.
   I never had a teacher like that, but I think it is a test 8 any teacher could and should give. Students who are not 9 in what they have learned are B students at best(充其量).
   The same is 10 in real life. The A students are those who believe in what they are doing because they have 11 both successes and failures. They have learned life’s lessons, either from normal education 12 from events in their lives, and have become 13 people.
    Scientists say that by the age of eight, 80% of what we believe about ourselves has already been formed. You are a big kid now, and you 14 that you have some limits (限制). However, there is 15 you can't do or learn or be. Sir Edmund Hillary, the first person to reach the summit of Mount Everest said, “It’s not the mountain we conquer(克服) but ourselves.