18. There is a widespread belief in the US and Western Europe that young people
have a smaller commitment to work and a career than their parents and
grandparents and that the source of the change lies in the collapse of the
'work ethic'.

A. a smaller commitment to work and a career than their parents and grandparents

B. less of a commitment to work and a career than their parents and grandparents

C. a smaller commitment to work and a career than their parents and grandparents

D. less of a commitment to work and a career than their parents and grandparents
had

E. a lessening of the commitment to work and a career than their parents and
grandparents had

答案是 D

我有一个疑问，这里的动词补出都用的是had,

因为前面不是完成时，而是是适宜动词，所以我觉得应该用did

不过既然答案是D，我就想问一下，动词的补出是可以补出原动词的吗？

例如

children eat more foods than their parents eat

children eat more foods than their parents do

哪句对呢？还是都可以？

当然可以补出实意动词。

两个例句都对！

这里的had是实意动词不是助动词，因为主句里有实意动词have对应had所以不会产生had是助动词的误解。

个人意见，仅供参考