【请教】GMAT prep一道语法题

25. 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，我选了B，就差了最后的had。语法笔记中说缺少助动词had会产生比较歧义，than前面有不止一个名词。但是我认为前面就是一个实义动词had呀，就算是要补充，也应该补充did呀。不明白，请教各位NN：）

如果than后面不补出动词的话 比较对象就不对等了 成了work and a career than their parents and grandparents

补had我认为其实是对应have a smaller commitment to work and a career（ 选项里面只有补had）

问是否其他实义动词根据时态补did代替？

后来又看过以前的讨论贴，是说其实要看怎么理解had了，可以理解为实义动词，也可以理解为助动词，所以应该是补充had和did都可以的，但是必须补充。