Q11 - Human skin gives off an array

[mshinners]

Question Type:
Necessary Assumption

Stimulus Breakdown:
Two gases given off by humans (gross) don't attract mosquitoes. Mosquitoes also find humans in the dark. Therefore, mosquitoes like some other human gas (gross, again).

Answer Anticipation:
Why is this author so hung up on gases? While it doesn't appear mosquitoes can see humans, maybe there's a factor other than gases at play. We should look for an answer that rules an alternative factor out.

Correct answer:
(B)

Answer choice analysis:
(A) Even if they do, they could communicate our location based on gases. "Hey, Brzzleby, there's a smelly human over there that's delicious!"

(B) Boom. It might not be gas - maybe it's heat! Since this answer rules out an alternative explanation, it's our answer. If we negate it and heat is how mosquitoes find humans, our argument falls apart.

(C) As long as enough gases are given off for a mosquito to smell a human, the relative levels don't matter.

(D) Even if they can navigate to Chez Human in darkness than in light, gases could still be at play (maybe the light affects the gases somehow).

(E) Way too extreme! Maybe there are some mosquito proof humans, but the rest off us give off delicious mosquito fumes.

Takeaway/Pattern:
When a conclusion settles on an explanation, the correct answer will usually deal with an alternative.

#officialexplanation

[NathanH184]

I guessed A. I was thrown off because the passage mentions nothing about body heat. I suppose that the answer doesn't always have to be "relevant" or "consistent" with the passage. My question then is, how does one know when to think about an alternative explanation when taking the test? Are there only certain questions when using this approach?

[DPCTE4325]

Does the negated version of answer choice D Fail wreck the argument because it doesn’t provide an “alternate explanation” the same way answer choice B does?

I picked B but am a little confused with the impact of negated answer choice D. If “Mosquitoes ARE more successful in finding a bare human arm in darkness than in light”, then couldn’t this be the reason why a bare human arm attracts more mosquitoes?

[ohthatpatrick]

We're trying to figure out
Why do mosquitoes prefer a bare human arm, which emits CO2 and lactic acid, to just raw quantities of CO2 and lactic acid?

i.e. What ELSE is there about an arm, in addition to the CO2 and lactic acid, that makes mosquitoes more attracted to it?

negated (B) says, "The bare arm ALSO has body heat, which mosquitoes like"

negated (D) says, "They're better at finding an arm in the dark than in the light"

Okay .... I don't care HOW GOOD THEY ARE at finding the arm. I care about why they're seeking it out in the first place. Why do they like the arm? (D) doesn't give us any reason why they like arms.

[CharlesT757]

I'm just trying to figure out how to negate answer choice (E). Would never change to sometimes? I am thinking along the lines of none negates to some.

[Laura Damone]

Exactly! Never negates to sometimes. And sometimes only necessarily implies "at least one time." For that reason, I always negate "never" as "it happened once." If that one occurrence isn't enough to break the argument, the answer is wrong! You can do the same thing with any statement of totality, positive (ie. All, Always, etc.) or negative (ie. none, never, nowhere). If one outlier doesn't destroy the argument, the answer is wrong. Hope this helps!

[LawrenceR550]

This was a question where I initially eliminated all choices.

We're trying to figure out
Why do mosquitoes prefer a bare human arm, which emits CO2 and lactic acid, to just raw quantities of CO2 and lactic acid?

i.e. What ELSE is there about an arm, in addition to the CO2 and lactic acid, that makes mosquitoes more attracted to it?

negated (B) says, "The bare arm ALSO has body heat, which mosquitoes like"

negated (D) says, "They're better at finding an arm in the dark than in the light"

Okay .... I don't care HOW GOOD THEY ARE at finding the arm. I care about why they're seeking it out in the first place. Why do they like the arm? (D) doesn't give us any reason why they like arms.

Negated (B) says "Mosquitoes are attracted to humans by body heat." Well why can't they be? We know these gases attract them, and they might be the preliminary form of attraction, but that body heat can't also attract mosquitos? I would say:

Okay .... I don't care HOW POWERFULLY GASES ARE at attracting mosquitos, in comparison to other potential attractors. As long as it doesn't negate the fact that they do, it's still possible that heat attracts mosquitos.


As for D, even though it might not give us any reason why they like the arm, if they are indeed better at finding the arm in the dark than in the day, that suggests that, in spite of the visual cues not being there (particularly because of that reason), there could be another explanation (say, that they communicate) which would in turn prevent the author of the stimulus to conclude that it's all about gases.

I suppose there really must be one reason (gases) for the conclusion to be drawn.