用所给词的适当形式填空。

1. —You look ______ (worry). What's happened? 

    —Oh, I can't find my English book.

2. Two ______ (visit) have just arrived, they are ______ (America).

3. —How long ______ you ______ (study) English? 

    —For two years.

4. They              (not hear) of that thing yet.

5. —When              he              (return) it? 

    —Half an hour ago.

6. ______ (thank) to your help, I can find my dog.

7. —______ he ______ (be) to Beijing before? 

    —Yes, he has.

8. I ______ never ______ (see) such a wonderful film before.

9. I happened _______ (meet) my old friend in the street yesterday.