Most of the thinking and/or discussion about this is right on, but I would warn you off using the word "inference" here to describe the incorrect answers. Generally when the GMAT uses the word "inference", it means what MUST be true, in which case the wrong answers here are emphatically NOT infer-able.
Conclusion: This is a draw a conclusion question, so there isn't one just yet.
Premises: LH have higher % allergies than RH.
LH better on right hemisphere, like math.
Draw a conclusion questions, as people have noted, always stay VERY close to the premises. Don't make any assumptions.
Left-handed persons suffer more frequently than do right-handed persons from certain immune disorders, such as allergies. Left-handers tend to have an advantage over the right-handed majority, however, on task controlled by the right hemisphere of the brain, and mathematical reasoning is strongly under the influence of the right hemisphere in most people.
(A) Most people who suffer from allergies or other such immune disorders are left-handed rather than right-handed.
PROBLEM: We know that LH "suffer [allergies] more frequently", but there are probably far more RH people, so they could still end up with a higher RH population with allergies (though not a higher %).
(B) Most left-handed mathematicians suffer from some kind of allergy.
PROBLEM: No connection is drawn between our two premises here.
(C) There are proportionally more left-handers among people whose ability to reason mathematically is above average than there are among people with poor mathematical reasoning ability.
ANSWER: Because this sticks to proportions (unlike A), it's our best candidate. Right brain people will make up the majority of people strong on math, and we've already been told that there are a higher proportion of LH in the right brain population.
(D) If a left-handed person suffers from an allergy, that person will probably be good at mathematics.
PROBLEM: Same as B.
(E) There are proportionally more people who suffer from immune disorders such as allergies than there are people who are left-handed or people whose mathematical reasoning ability is unusually good.
PROBLEM: Here, the issue of proportion is used incorectly. We don't know what proportion of people suffers from immune disorders IN GENERAL, only that the proportion of LH is higher than the proportion of RH.
Hope that helps!
Tommy Wallach | Manhattan GMAT Instructor | San Francisco
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