The problem is too difficult for him. He solve it by himself without any help.
A. can't
B. mustn't
C. shouldn't
D. needn't
网友回答
(答案→)A. can't
B. mustn't
C. shouldn't
D. needn't
解析：can't不能，不可能；mustn't禁止；shouldn't不应该；needn't不必。句意：这个问题对他来说太难。没有帮助他自己不可能自己解决。故选A。