PREP上一道语法题关于less 的用法求解- -

熊的传人

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."C

(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
这题正确答案选D， 但是D中 a commitment 不是单数可数名词么 为什么可以用less 来修饰？ less 不是只能修饰不可数么。。。