Answer:
introduce alternative's to Andrew Gallup's theory
Explanation:
We know A is wrong because in the last line, it mentions "If the function of yawning is to cool my brain, why do I do it more often when I see other's yawning?" Which implies that the author disagrees with this idea.
B is wrong because the point of this paragraph isn't to explain the difference but to just provide extra information about how they relate/don't, but this isn't the main idea.
C kind of sounds right, however, I still think D is right because the paragraph starts off with "Gallup's work is disputed; even the commenters on Grriscientist's post point out that there are other, simpler explanations for the trends shown in the research." Disputed means to disagree or to argue. This means that the commenters disagreed with the research. However, in the last line, it does ask us "If the function of yawning is to cool my brain, why do I do it more often when I see other's yawning?" which is kind of hinting that yawning may be contagious, but this still isn't the MAIN idea, therfore in my opinion, the answer is D
