从每题所给的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．Only when you stay and live with local people for a period of time will you have
better understanding of life here.
A．a; / B．the; a C．/; the D．the; the
