Logical reasoning PrepTest 146 · Section 2 · Question 11

Incorrect. Mosquito communication is out of scope of what attracts them. They could be attracted to human skin because of other released gases even if they can't communicate that attraction to other mosquitoes.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Carbon dioxide and lactic acid both attract mosquitoes and are released by human skin. However, human skin attracts more than either or both of these chemicals, even in the dark. So humans must also give off another gas that attracts mosquitoes.

Answer Anticipation:
The stimulus compares the attractive properties of certain gases to human skin and finds that human skin is more attractive to mosquitoes. That's just the background! From there, the rest of the argument is dedicated to explaining this phenomenon. It does rule out one potential explanation—human skin looks tasty to mosquitoes. However, from there, it concludes that it's another gas that human skin releases that provides the extra level of attraction. There are any number of non-gaseous explanations for the higher level of attraction mosquitoes have to human skins over the two named gases, and this argument assumes that none of them serve as the explanation. The correct answer will likely rule out at least one of these alternative explanations.

Answer Explanation:
This answer choice rules out an alternative explanation for the attraction, so it's the correct answer. If the mosquito attraction can be explained with body heat, then there's no longer a phenomenon that needs the explanation of other attractive gases.

Key Takeaway:
When the conclusion in a Strengthen with Necessary Premise question settles on an explanation for a phenomenon, the correct answer will generally rule out at least one alternative explanation. This can be tricky since these alternatives can feel out of scope because they're unmentioned in the stimulus. But much like alternative causes are in scope of an argument that concludes causality—even when that alternative isn't mentioned in the stimulus—alternative explanations are in scope of an argument, even when that explanation isn't mentioned.

Incorrect. This answer is trying to get you to make a jump about day versus night based on the part of the study that was done "in complete darkness." However, night isn't completely dark, and this answer requires too many jumps to be correct.

Incorrect. Finding the arms isn't the relevant consideration—being attracted to the arm is. It's possible that the mosquito attraction can be measured even without actually finding the arm (maybe they buzz differently).

Incorrect. Negating this test to see if it's necessary really helps to eliminate it. If human skin every once in a while gives off a gas that repels mosquitoes, it could still give off attractive gases the vast majority of the time.