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．—May I have a look at your books?
—__________.
A．By this means B．By all means C．In this way D．By any means
22．It's very important for a student to __________ a good state of mind while taking the college entrance examination.
A．keep out B．keep on C．keep up D．keep off
23.The new machine, if __________ properly, will work for at least ten years.
A. use B. using C. being used D. used
