Took me a while but C is it. Again, here is a number example to prove my case:
Key sentence: Left-handers tend to have an advantage over the right-handed majority
We know from above that there are more right handed people than left handed people.
Let's say there are 90 right handed people and 10 left handed people for a total of 100.
Left-handers tend to have an advantage over the right-handed majority in terms of mathematics which is largerly controlled by the right part of the brain
Hence, let's say that 60% of left handers are above average in mathematics and 10% of right handers are above average in mathematics. This satifies the above premise and translates to:
6 left handed person good in math
9 right handed persons good in math
There are proportionally more left-handers among people whose ability to reason mathematically is above average
The above means 6/ [9+6] = 40%
than there are (left handers) among people with poor mathematical reasoning ability.
The above means [10-6] / [4+80]
= people not good in math
Above two concur with what C means. As a matter of fact, this relationship can NEVER be violated and will ALWAYS hold true. Hence, C MUST be the answer