need some help on this

1.        According to a study by the Carnegie Foundation for the Advancement of Teaching, companies in the United States are providing job training and general education for nearly eight million people, about equivalent to the enrollment of the nation’s four-year colleges and universities.

(A) equivalent to the enrollment of

(B) the equivalent of those enrolled in

(C) equal to those who are enrolled in

(D) as many as the enrollment of

(E) as many as are enrolled in

The answer is E, could any one hint out why E? to me, A and D look good. the only reason i can get out of my head is they are matching "people" to "people", not number to number. Need help on this. Thank you !

equivalent主要用于不可数名词，而且偏重抽象名词。

D的话是不平行的 people 不等于 enrollemnt,　E省略了 those 即 as many as (those) are enrolled in