第一部分；单项填空（每小题1分,满分15分）
21．It is ______ challenge for graduates to find ______ work as unemployment is very high nowadays.
A. the; / B. a; / C. a; a D. /; the
22．—Excuse me, I wonder if you can help. I ______ my ID card.
—But where did you lose it?
A. have lost B. lost C. had lost D. lose
23． —Why didn’t you go to Mike’s birthday party yesterday?
—Well, I ______, but I forgot it.
A. should B. would C. should have D. must have
