31. You see the lightning _____ it happens, but you hear the thunder later.
Although he knows clearly that smoking harms his health, he simply cannot __________.
There is no doubt that by recycling textbooks, the cost of books will be __________ greatly.
I agree with most of what you said, but I don't agree with __________.
—Do you know the man __________ is talking with your father?—Yes, he's our headmaster.
__________ David says sounds right to Helen. That's why she has made up her mind to leave with him __________ happens.