Question

While crossing the street, _________．

1. A.
   the light must be green
2. B.
   there are no policemen
3. C.
   be careful
4. D.
   it is necessary to be careful

Answer 1

C
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．