6. By the time I got to the airport, the plane ________.

　　　 A. took off　　　　 B. has taken off　 C. had taken off　　 D. takes off

5. He ________ English for 6 years before he went to college.

　　　 A. has learned　　　 B. had learned　 C. learned　　 D. learns

4. By the time he was fourteen, he ________ up a small chemistry lab.

　　　 A. had set　　　　 B. has set　　　 C. set　　　　　 D. sets

3. He ________ a composition before he attended the meeting.

　　　 A. had written　　 B. has written　 C. wrote　　　　 D. writes

2. The people heard the bad news that the hero ______ several hours ago.

　　　 A. had died　　　 B. has died　　 C. died　　　　 D. dies

1. I ______ all the exercises before our teacher collected the exercise books.

　　　 A. have finished　 B. finished　　 C. had finished　 D. finish