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问题 多项选择题

某物体做变速直线运动，其加速度方向不变，大小逐渐减小到零，该物体的运动情况可能是（）

A.速度不断增大，最后达到最大，并以此速度做匀速直线运动

B.速度不断减小，最后达到最小，并以此速度做匀速直线运动

C.速度不断减小，又不断增大，最后做匀速直线运动

D.速度不断增大，又不断减小，最后做匀速直线运动

答案

参考答案：A, B, C

解析：若物体做加速运动，速度方向与加速度方向相同，加速度减小，速度仍然增大，当加速度减小为0时，速度最大，而后做匀速直线运动．故A正确，D错误；当加速度方向与速度方向相反，速度减小，物体加速度减小为零时速度达到最小，而后做匀速直线运动，故B正确；当加速度还未减小到零，速度已减小到零，则会反向做加速运动，当加速度减小到零，做匀速直线运动．故C正确

单项选择题

After a day’s hard work, we need some sleep. During the sleep, the fatigue (疲劳) of the body disappears and recuperation (恢复) begins. The tired mind gathers new energy, and the memory improves: Some adults require little sleep; others need eight to ten hours in every twenty-four hours. Infants (婴儿) sleep sixteen to eighteen hours dally, the amount gradually diminishing as they grow older. Young students may need twelve hours; university students may need ten. A worker with a physically demanding job may also need ten, whereas an executive working under great pressure may manage on six to eight. Many famous people have little sleep. Napoleon Bonaparte, Thomas Edison and Charles Darwin apparently averaged only four to six hours a night.

Whatever your individual need, you can be sure that by the age of thirty you will have slept for a total of more than twelve years. By that age you will also have developed a sleep routine: a favorite hour, a-favorite bed, a favorite position, and a rule you need to follow in order to rest comfortably.

Investigators have tried to find out how long a person can go without sleep. Several people have reached more than 115 hours—nearly five days. Whatever the limit, it is absolute. Animals that kept awake for from five to eight days have died of exhaustion. The limit for human beings is probably about a week.

If a person is not allowed to follow his own sleep routine, he will probably（）

A. never fall asleep

B. not sleep well

C. lose his memory

D. begin to daydream
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单项选择题

对1929～1933年经济危机的说明中不正确的是( )。

A．从美国纽约华尔街股市崩溃开始
B．根本原因在于资本主义的基本矛盾
C．波及整个世界
D．对世界局势产生了巨大的消极影响
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