On our increasingly frequented Forums, there’s been a recent spate of posts in which well-meaning users have posted official problems from GMATPrep – usually sentence correction problems – and then questioned or decried the construction of the correct answer choices.
“Is X really allowed? Isn’t it supposed to be Y?”
Some of these posters have actually gone to the trouble of looking up the disputed constructions in sundry reference works, including dictionaries and style guides, to try to find ammunition with which to attack the officially correct answer.

Never forget the following iron law:
On official problems, CORRECT ANSWERS ARE CORRECT, in every possible way.

This fact may sound obvious, but many students don’t realize its full consequences: namely, that every grammatical construction found in a correct answer to an official SC problem is officially valid, that every idiomatic expression in such an answer is correct, and that every word choice in such an answer is appropriate.
ALL of them.

This is an inviolable fact. Remember that the GMAT is a dictatorship, a consensus of one: only GMAC ultimately makes the decisions about which grammatical and idiomatic constructions are acceptable and which aren’t. We’re all playing on GMAC’s playground, and GMAC makes the rules.

What this means for you, the student, is that it’s a complete waste of time for you to question any official answer to a problem published by GMAC. Indeed, the only appropriate response to a correct answer that you find surprising, illogical, or “ugly” is this:
“Wow, that’s unusual. I guess I’ll have to recalibrate the way I think about that, because now I know I can do _____.”
Again, this is the ONLY way to respond to surprising constructions, solutions, and so on in officially correct answers. If you respond by questioning or doubting the validity of such answers, or, worse yet, actively trying to dispute that validity, then you are at best sidelining your studies with needless detours, and at worst confusing yourself.

On official problems, correct answers are correct.
That’s it.

There may be answers that you don’t like – I, for one, have been positively disgusted by a few of the officially correct SC answers I’ve seen – but you’ve got to learn to play by GMAC’s rules.