Their dog was a substitute ___ the children they had never had.

问题描述:

Their dog was a substitute ___ the children they had never had.
a:as
b:of
c:to
d:for
感觉a和d比较像.是哪个呢?-.-
另外 never had是个固定词组吗?还是 never have也可以?

我认为是d,因为substitute可以跟for做成词组