此题应选D.
E错在缺少了verb. Let's put aside the argument of "charge" vs. " those charging" first, E can be extracted as "more than three times as many A as B", it becomes a noun cluster, not a complete clause.
D is correct, "as charge" omits "institutions", and "charge", working as verb, agrees with the plural subject. So let's substitute D back to the whole sentence: According to XX study, more than three times as many institutions charge fees under $8000 a year as (institutions) charge over $16000. |