Excerpt from Stacey - Manhattan GMAT forums
First, it shouldn't be "help the listener to determine" (although that is how lots of people would say it). You don't need the "to" - and, if you don't need it, then it's not going to be in the correct answer!
Be careful about deciding based upon what sounds good or bad. The test will fool your ear. Go based upon the specific rules you KNOW are true.
You may read an original sentence and think it sounds awkward (in fact, this will happen quite a lot!), but if you cannot point to a specific area that you KNOW is an actual grammatical error, don't eliminate A. It stays in as a possibility.
The in vs. between issue is an idiom. I say differences in X (just one thing) or differences between X and Y (two things). This sentence has one thing: how the two ears hear a given sound. So, I need "differences in." Elim D and E.
B says "differences in the two ears hearing" - that makes it sound like the two ears themselves are different (as in, they look different or something), as opposed to a difference in the way the two ears perceive a sound. That's not the original meaning (and doesn't even make a lot of sense), so elim B.
C says "differences... helps" - that's a subj-verb mismatch. Elim C. (you can also use this to elim D, if you haven't already eliminated it).
Only A is left. (And, usually this will be the process for getting yourself to A. There won't be anything wrong with it but you'll be suspicious of it b/c 80% of the time there IS something wrong with it, so you'll find some reason to say it sounds bad. But DO NOT eliminate A unless you can point to a specific error. Leave it in and test the others.)
Aiming High
