—Many houses near the airport were destroyed by the wind.—Well, the wind ________ have been very strong.
34. When he knocked at the door, my mother was busy ____ supper.
33. He waited until it was dark ____ nobody could see him clearly.
31. We should think of ways to stop the waste from the factory _____ our living environment.
28. As far as I know, this man ____ risk anything than lose the ring for his wife.
27. She locked her room _____ a thief would break into it.