从题中所给的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．It is a set rule that all payments ______ be made by the end of the month.
A．will B．shall C．can D．may.
22．Heavy snows ___ Dalian on Jan 4,2010, causing great damage to the people there.
A．beat B．hit C．happen D．Occur
23. The term "weak eyesight" is misleading, _______ in near-sighted eyes the lens (水晶体) of the eye is actually too strong.
A. though B. for C. so D. unless
