Because the budget package in Congress promises a combination of higher taxes and reduced spending that may slow economic growth, many in the credit markets wonder whether the Federal Reserve will compensate and help the economy by keeping interest rates low, or perhaps even if they push them lower.

A. by keeping interest rates low, or perhaps even if they push
B. by its keeping interest rates low, or perhaps even by their pushing
C. by keeping interest rates low, or perhaps even by pushing
D. through their keeping interest rates low, or perhaps even push
E. through their keeping interest rates low, or perhaps it will even push

I thought that the needs to be a parallelism - Whether X or Y.
Similarly Whether the Federal Reserve 'will compensate' OR perhaps 'will push'. I thought it will be between D and E. However the OA is C. Please guide.

Ron and Manhattan experts,
I would very much like know the difference between the use of "by" and the use of "through".

please clarify me and thank you very much.

wrong parallelism. there is no parallel structure of the form “whether x or y” in this sentence, because people are only wondering “whether” one thing will happen: namely, they are wondering whether the federal reserve will compensate xxxx blah blah blah.

there should be parallelism, however, between the two ways in which the federal reserve might accomplish this goal:
1/ they might keep interest rates low
2/ they might push rates even lower

therefore, the words that must be parallel are “keep” and “push”.

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes

most importantly, this sort of difference is extremely unlikely to be tested on the exam. as you will see, it's quite subtle -- well beyond the scope of anything that gmac could reasonably test. if you see this split in a real problem, it's almost certainly meant to distract you from something else.

--

nevertheless, in case you are still interested, here's a rough idea:

* if a sentence specifies, at least somewhat precisely, the method by which someone is going to accomplish something, then it should use “by”, not “through”.

* if a sentence mentions a very general category of approaches, then “through” is more appropriate.

here are two examples:
i got to know james better by asking him questions about his life
(note that “asking him questions about his life” is pretty specific)
i got to know james better through conversation
(“conversation” is much more general)

_________________
Being well-dressed gives a feeling of inward tranquillity [that] religion is powerless to bestow.
C.F. Forbes