This is a question of recognizing the correct idiom for this comparison and then applying the rule of concision.
The idiom
"X times as many Ys do such-and-such" is completed by another phrase beginning with
"as", not by "than". There is no use of comparative words ("better", "stronger", etc.) or use of the words "more" or "less", which would dictate the use of "than". This is one of those situations where it's all too easy to conflate idioms, because there IS a comparison made here, so the mind tends to supply "than". But the comparison is not made with the use of comparative words requiring "than". Once you recognize the correct idiom, you can eliminated choices A through C.
After that, it's a matter of recognizing what's being compared and the most concise way of expressing that. What's being compared:
independent institutions of higher education charge tuition and fees of under $8,000 and
[independent institutions of higher education charge tuition and fees] over $16,000. To find the most concise way of comparing, strip way everything not common to both sides of the comparison. Here's it's
of under $8,000 versus
over $16,000. Choice D is more concise and does a better job of comparing "apples to apples", because what's being compared is
not just two subsets of institutions but
the actions of two subsets of institutions (i.e., how much they charge). Furthermore, choice E would introduce pronoun ambiguity with the use of the plural demonstrative pronoun "those", which could refer to "institutions" or to "tuition and fees".
So, to reiterate, choice
D is best.
Looking at other uses of this idiom will help clarify:
- * "Twice as many Americans watch football as watch baseball." You want the verb in the comparison to make it obvious that what's being compared is the object of watch and not the subject. Compare to the next example, where subjects are compared.
* "Twice as many girls entered the Science Fair as did boys." Or: "Twice as many girls as boys entered the Science Fair."
* "Half again as many Americans voted for candidate A as for candidate B." The idiom "half again as many" means 1.5 times. Notice that you do not need to repeat the verb because the preposition "for" makes it obvious what's being compared. You could not say, "Half again as many Americans voted for candidate A as candidate B," because this wording doesn't observe parallel construction, creating ambiguity whether "candidate B" is being compared to "Americans" or to "candidate A". Another example: "Twice as many bird species court by visual displays as by vocalizations."
* "One-fifth as many flowers self-pollinate as cross-pollinate." Here it's actions that are being compared. Also: "Twice as many runners pronate as supinate."
