Look at the mess the boys made! You have taught them how to behave themselves.
A. must
B. may
C. can
D. should
网友回答
A. must
B. may
C. can
(答案→)D. should
解析：考查情态动词。must 必须，may 可能，can 会，should 应该，句意：看孩子们弄得一团糟！你本应该教给他们如何守规矩。should have done 本应该做某事而实际上没有做，所以选D。