用括号内所给单词的适当形式填空

1. It's five o'clock. The students __________(play) football on the playground.

2. You __________(not pass) the exam if you don't work hard enough.

3. The baby is sleeping. Could you please stop __________(make) so much noise?

4. This machine __________(use) for cleaning the house.

5. The man spoke very slowly. He wanted me __________(understand) what he said.

电灯悬挂于两墙之间，如图所示，使接点A上移，但保持O点位置不变，则A点上移过程中，绳OA的拉力(     )

A．逐渐增大

B．逐渐减小

C．先增大，后减小

D．先减小，后增大