While crossing the street, _________．A.the light must be greenB.there are no policemenC.be carefulD.it is necessary to be careful
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C解析题意是“过大街时，一定要小心”。当主句的主语和从句的主语相同时，从句可以改为引导词加分词形式，反过来，从句是引导词加分词形式，主句必须和从句保持主语一致。C项是祈使句，其主语是you。该句可改为You must be careful while you are crossing the street．