crossthestreetwhenthered light is on

We cross the street when the red light is on.Obcy the traffic rules,children.(  )

A. can

B. mustn't

C. may

D. shouldn't

答案:B.
can,能,会;mustn't不可以;may可以;shouldn't不应该.根据when the red light is on,可知根据生活实际,红灯亮时,我们不能横穿马路,所以这里用mustn't来表示禁止.故选B.
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