求教语法题，非常谢谢

非常抱歉，XDF练习，不知题号

题干：According to a study by ....., companies in the US are providing job training and education for nearly eight million

people,about (as many as the enrolment of)  the nation's four-year colleges and universities.

(A) as many as the enrollment of
(B) as many as are enrolled in

答案选B。不理解
个人认为about 介词不应跟句子，且B主语不清。

my two cents:

B 补全了是: .....nearly eight million people,about as many as (people) are enrolled in .... 

about is an adverb here.

if A were  as many as (the) enrollments in... ,  A would be OK.

还是疑惑

在牛津高阶字典查到“The school has an enrollment of 800 pupils”例句

个人认为如用A，则“，about”后是个名词短语，

B中少people，少主语，即成分不全
 
为一解释是 同位语无法修饰次要成分，这个问题比少主语更严重