[求助]

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

答案是D, 我选E, 后来我想charging 根基平行原则的确不妥，但请问D的这种省略主语的表达有什么根据吗？谢谢各位