括号中所给动词的正确形式填空。

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.

为什么第二阶段是生产要素的合理投入区间？