求助关于助动词的补充（prep2和大全181）

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

我认为官方OG当中把have、had一只作为实义动词来做，所以助动词只能是do、does，did，但是如果初中出来的助动词是had或者have，那么就是完成时态的用法。。

按照这个正确答案，我认为C中的had不应该是过去完成时态的助动词（因为没有明显的表示是发证在过去的过去），而应该是系表动词的用法，和Be的用法一样。。

但是下面这题：