Hi jona,
First, if you haven't already done so, I'd recommend reading Adam's (Post #8) and Jeremy's (Post #10) earlier posts for this question.
They can be found here:
viewtopic.php?f=1224&t=14754
As they mention, the stimulus here contains a causal argument. Assumption questions with causal arguments usually involve a Defender assumption that protects the argument from one of the several ways of attacking a causal argument, most often by eliminating a possible alternate cause.
That is exactly what Answer B does. Using the Assumption Negation Technique, if mosquitos are attracted to humans by body heat, that is enough to undermine the conclusion that human skin gives off some other gaseous substance that attracts mosquitoes since there is no longer any support for this conclusion.
While I understand your concern that the level of attraction would need to be sufficient to be relevant, the general wording "mosquitoes are attracted" without any modifiers would indicate that sufficiency/relevance. In plain English/common usage, the phrase "mosquitoes are attracted to ...." (without any modifiers) would mean a statistically significant increase, not some very minor attraction.
For example, if I state that "sharks are attracted to blood," the general understanding of that phrase doesn't include that sharks may be so weakly attracted to blood so as to be virtually negligible. While it would probably be more accurate to state that "sharks are very attracted to blood," the lack of a modifier doesn't make the original statement incorrect.
