用括号中所给词的适当形式填空。

1. I haven't decided what ________ (write) about.

2. I often hear him________ (sing) in the next room.

3. I find it easy _______ (work) out the difficult problem.

4. It is too dark to see the blackboard________ (clear). you had better turn on the light.

5. — Where is Bob?do you know? — Oh, he                (go) to the post office.

6. The teacher is ________ (please) with what we have done in class.

7. Tom ran so ________ (quick) that I couldn't catch up with him.

下列选项中，属于关税措施的是（ ）

A．配额

B．许可证

C．从价税

D．外汇管理