求助GWD24-20，没搞明白啊╮(╯_╰)╭

crystalhuang

Q 20:
According to a 1996 survey by the National Association of College and
University Business Officers, more than three times as many independent
institutions of higher education charge tuition and fees of under $8,000 a
year than those that charge over $16,000.

A.    than those that charge
B.    than are charging
C.    than to charge
D.    as charge
E.    as those charging
为什么选E
我觉得应该是charge啊，为什么是ing形式，能不能给我解释下？？········

AlohaDJ

(1) as many...as

(2) charging是V+ing作后置定语，修饰those，逻辑上和those构成主谓关系。