关于enroll的一道题。。。

ll233.
                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）

(E) as many as are enrolled in

enrollment是入学人数，不可能等于8million，但是答案E里面enrolled in为什么就可以等于800万了呢？不太理解

这里是主语比较省略了第二个主语的情况,完整的情况应该是

as many as (people) are enrolled in the...