从 A、B、C、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. –Shall I lock the lab now before I go home?
--____________. I’ll check it myself later.
A. Go ahead B. No problem C. No hurry D. Don’t bother
22. At _____ young age, Andy took up a part- time job and got paid by _______ hour.
A. / ; / B. a ; / C. a ; an D. a ; the
23. She loves giving parties, and does so whenever she can find ________.
A. an excuse B. an apology C. an explanation D. a cause
