求助：Gmatpre一个比较结构的题

下面这题来自于gmatprep:

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。why it is "had"?? I think it should be "did". NN helps me. I do appreciate!