请教一道没人问过的，prep 2-25百思不得其解

There is a widespread belief in the United States 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 that of 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 that their parents and grandparents had

这道题答案是D，我选了B

我觉的答案中的助动词补错了，have sth 此处have是实义动词，补出助动词应为do,此处就应该为did啊

如   I have an apple, do you?

 若是have done sth, 那助动词就是have :   I have seen this movie, have you?

 此外，have to do 助动词也是do

为什么这道题是D呢？

请大家指教！！

另如果是have sth done,这种助动词是have 还是do?

谢谢大家！