从A到D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
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．---Helen, will you be at the party tonight?
---Yes, but I really_________ as I have so much homework to do．
A．can’t B．shouldn’t C．mustn’t D．won’t
