题干

在平面解析几何中,当动点到一个定点的距离与它到一条定直线(定点不在定直线上)的距离之比是常数时,该动点的轨迹为圆锥曲线。常数的值不同,圆锥曲线的形状就不同。当常数小于1时,轨迹是椭圆;当常数等于1时,轨迹是抛物线;当常数大于1时,轨迹是双曲线。上述结论说明(   )
①共性寓于个性中                  ②矛盾的同一性推动事物的发展 
③事物的量变引起质变              ④事物的联系是具体的,多变的

A:①②③

B:②③④

C:①②④

D:①③④

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答案(点此获取答案解析)

D

同类题5

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项,并在答题卡上将该项涂黑。选项中有两项为多余选项。

      Sleep is something we all do. But some people need to sleep more than others. Babies sleep most of the time. Children in school sleep about ten to twelve hours a night. Most adults sleep only seven or eight hours.____All parts of our bodies have to rest after they work.. Our arms need a rest after we lift heavy thing, When we run fast, our legs work hard. They get tired. We have to rest them. Our brains workhard, too.____ We can sit quite still and rest our arms andlegs. But our brains aren't resting. They go right on thinking as long as weare awake.

      Our brains slow down a bit when we sleep and dream.____ Instead of thinking wide-awake thoughts, our brains make up dreams. Some dreams are very pleasing. Some are not. Most of thetime we forget them when we wake up.

      Scientists have tried to find out what would happen if people were not allowed to sleep. They asked some people not to go to bed. The people stayed up all night and all the next day. They stayed up the next night too,and the day after. They played games, but they made mistakes. They forgot things.____ The people grew rude and mean. They became angry with their friends. Finally they were too tired to stand up. When they sat down, they fell asleep.

      Scientists have found that if people are not allowed to sleep and to dream, they act in an unusual way.____But we do know that we need it to stay well. So tonight have a goodsleep. Lie down under the covers. Shut your eyes. Let your thoughts wander.Soon you’ll stop thinking. You'll be asleep.

A. It was hard for their tired brains to work.

B. When we are awake, they help us pay attention to the worldaround us

C. But babies, children, and adults----all of us need to haveour sleep

D. Good sleep helps to improve one's memory

E. No one knows why sleep is so good for us

F. But even as we sleep our brains are doing some work

G. Of course you will have a good sleep