选词填空。

1. They _____________ (is / are) playing.

2. She is _____________ (sit / sitting).

3. — What's the matter?  —— My head _____________ (hurt / hurts).

4. He is _____________ (eating / eats).

5. What are they _____________ (do / doing)?

用户不能直接调用构造方法，只能通过______关键字自动调用。