请教一道prep题目

langrenly

3.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
不明白为啥C不行？答案说是that of会导致比较对象不一致，可是that指代commitment 的话，我觉得没问题啊，是不是前面的a smaller修饰有问题呢？

blueeye16

young people have ...than their parents and grandparents ...
sb have...than sb had...

langrenly

多谢两位！