Sentence Correction

Original question:

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A new hair-growth drug is being sold for three times the price, per mg, as the drug's maker charges for another product with the same active ingredient.

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choices: as that of what at which

What grammar point is being tested here?


When do we say "twice/three times/x times as much/many as"? and when do we say we say "twice/three times/x times that ..."?


Thanks very much,

J