You__him so closely;you should have kept your distance.

问题描述:

You__him so closely;you should have kept your distance.
为什么不能选shouldn't follow
而要选mustn't follow

mustn't follow表示"不允许,不准".用了就表示没有回旋余地的不能去做; shouldn't follow 表示"不应该"意思不够强烈,也就是说虽然"不应该"但做了也是允许的.
you should have kept your distance.确定了这是不被允许的,因此要选mustn't follow