求助：一道prep的题

25.   (25704-!-item-!-188;#058&002607)

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
        

答案是D，但我觉得had 也算不上是助动词吧，要补不是也是应该补did的么~~~