We talk loudly when we see the sign on the right

问题描述:

We talk loudly when we see the sign on the right
must mustn't need needn't
答案选mustn't ,谁可以帮解答

mustn't 表禁止.首先要读懂句子的意思——当我们看到右边的标识时,我们不能大声说话.
标志里有的是Don`t speaking 之类的.
must和need不符合句意,needn't表没必要.