21. You were silly not ____your car．(lock)

to have locked

20. ____in the queue for half an hour，Tom suddenly realized that he had left his wallet at home．(wait)

Having waited

19. My advisor encouraged me ____a summer course to improve my writing skills．(take)

to take

18. Don’ t leave the water ____while you brush your teeth．(run)

running

17. ____ by terrorists，the tall building collapsed．(attack)

Having been attacked

16. The disc，digitally _____in the studio，sounded fantastic at the party that night．(record)

recorded

15. The flowers _____sweet in the botanic garden attract the visitors to the beauty of nature．(smell)

smelling

14. The flu is believed ____by viruses that like to reproduce in the cells inside the human nose and throat．(cause)

to be caused

13. According to a recent U．S．survey，children spent up to 25 hours a week ____TV．(watch)

watching

12. “We can’t go out in this weather，”said Bob，____out of the window．(look)

1ooking