you mustn't.
b．can与be able to：can表示有能力做某事，be able to表示通过努力、克服困难做成某事。如：
Although his leg was hurt, he was able to swim to the bank，
c．can't和mustn't；can't多表示\没能力''或否定的推测，也可表示\不可以，不允许\等含义，而mustn't 表达\禁止，不允许\含义时更
常用，而且语气更强。如： Man can't live without air．
He can't be that foolish. Students mustn't smoke． d. can／could用于否定句或疑问句，表示\不可能是(在)……\或\可能是(在)……吗\。
You can't be serious． =You must be kidding/ joking.