Passage Summary: Mosquitoes are drawn to the CO2 and lactic acid we breathe out and sweat, but they are even more attracted to an actual arm than to those two chemicals alone. Since this happens even in the dark, the author thinks there is another hidden gas we are giving off that acts as a mosquito magnet.

Conclusion: There must be an additional gaseous substance emitted by human skin that attracts mosquitoes.

Reasoning: While CO2 and lactic acid attract mosquitoes, their combined effect is significantly weaker than that of a real human arm, even when visual cues are eliminated.

Analysis: The author is using a process of elimination to isolate a cause. By mentioning that the attraction persists in 'complete darkness,' the stimulus effectively rules out visual cues as the missing factor. However, for the conclusion to hold, the author must assume that no other non-gaseous factors—like body heat or moisture—are responsible for the extra attraction. Look for an answer that bridges this gap by ensuring the 'extra' attraction isn't coming from something other than a gas.