Question

To make our city more beautiful, rubbish ________ into the river.

1. A.
   needn't be thrown
2. B.
   mustn't be thrown
3. C.
   can't throw
4. D.
   may not throw

Answer 1

B
解析：
剖析：这是含有情态动词的被动语态。首先应排除C和D，因为它们是主动语态；
含有情态动词的被动语态的构成应该是：“情态动词+be+及物动词的过去分词”，needn't是“不必”=don't have to；
mustn't是“禁止，不准”之意，故选B。