She began teaching her three children at home.

题目

She began teaching her three children at home.
She began teaching her three children at home because she felt they weren't learning what she thought they _____ learn.
A.could
B.might
C.would
D.should
请问是不是选D?如果是那为什么不能选A或者C?
这里的should是表建议？

答案

should在此处相当于need表示应当学习的知识母亲怕孩子在学校学不到应该学到的知识才打算在家教.could would在句中只是表示可以可能将要,不符合句子意思