用所给动词的适当形式填空。

1. David often ___________ (help) me with my English.

2. Can you ___________ (put) an egg on your head?

3. Look, the farmers ___________ (pick) apples on the trees.

4. Nancy ___________ (play) the piano at the concert tomorrow.

5. The children went to the farm yesterday. They ___________ (have) fun there.

在做铁丝在氧气中燃烧的实验，在集气瓶里为什么要预先加少量水或铺一薄层细沙？为什么铁丝要绕成螺旋圈？为什么要系一根火柴，并且待火柴快燃尽之前插入集气瓶中？