Could an instructor kindly explain the following sentence which is correct:

Question 9 (correction): Mary buys cookies made with SugarFree, an artificial sweeter, which tastes as sweet as the corn syrup that her brother loves but which contains fewer calories than does an equivalent amount of corn syrup

Question:
(1) after the word "loves", should there be a comma? Does this function as a non-essential noun modifiers as per page 107?

(2) the words "but which" - is "but" a preposition?

(3) please explain if the words "but which" and "does an equivalent amount of corn syrup" - relate to correct parallelism?

Thank you.

(1) I think a comma would not be wrong before "but which." However, it is not required. Some writers use more commas than other writers, and comma usage is more hotly contended than other grammar issues. As a result, the GMAT will generally not expect you to make comma vs. no comma judgement calls, unless there is some other grammar rule that gets violated as a result. Comma vs. semicolon is fair game, though.

(2) "but" is a conjunction, like and, or, yet, for, so, etc...

(3) Yes to both.

a) The "which" after "but" is parallel with "which" after SugarFree/an artificial sweetener.
A quick paraphrase: SugarFree, which X (is sweet) but which Y (is low-cal).

b) The "does" is a proform (a word that references another, the way a pronoun does). Here, does = contains.

SugarFree, ... which contains fewer calories than does an equivalent amount of corn syrup.

means the same thing as:

SugarFree, ... which contains fewer calories than an equivalent amount of corn syrup contains.

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Emily Sledge
Instructor
ManhattanGMAT