Houses with low ceilings and thin walls were prevalent in North America even where there was no
Why this is right
Well, I am pretty opposed to this question overall, so take this explanation with a grain of salt. This answer is trying to take the form of a (No Cause, Effect) plausibility weakener. When it uses the past tense "were", we're apparently supposed to assume that it's talking about the period right after WWII. i.e., "Hey author, if A/C was really the cause of moving towards lower ceilings and thinner walls, then how do you explain that these houses were prevalent even where there was no demand for A/C?" (The author could simply say, "I just said they were the main cause. There were lower ceilings and thinner walls in areas w/o demand for A/C for some of those secondary reasons, such as they're cheaper to build and you can build more floors with lower ceilings.") This answer would only have the power the test writers are wanting it to have if it had said "houses with low ceilings and thin walls were just as prevalent where there's no demand for A/C". Or it would be better if it said, "Houses with low ceilings and thin walls were prevalent in new constructions after WWII even where there was no demand for A/C." Otherwise, it's really an answer with no impact. "Prevalent" just means "common / not rare". I'm sure that many styles of house are prevalent in all places. That doesn't mean they're desired or newly constructed or trending. Houses with stucco painted ceilings are very prevalent right now (because that was a fad in the 70s), but modern builders don't make new homes in that style. So even though modern builders have made a certain change in terms of how they paint ceilings in residential architecture, that old style is still prevalent in existing houses. Sigh, oh well. We can't fight city hall. I don't know why they think this answer is better than (E), which introduces an alternate explanation. But they think that this answer calls into question the plausibility of the author's explanation by saying that the observed effect (thinner walls / lower ceilings) was present even in areas not affected by the supposed cause (demand for A/C).
Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.