Critical Reasoning

Hi all,

I took the diagnostic test in OG 12 for Critical Reasoning, and found two questions in which the explanation seems a little strange to me. I don't really care if my reasoning or GMAT's reasoning is more "truthful" - instead, I want to figure out how the GMAT arrived at their answer, and what kind of assumptions they make, so I can keep these in mind for future questions.

Here's first question, the two answer choices I had it narrowed down to, and then the GMAT's reasoning of each one.

30) A physically active lifestyle has been shown to help increase longevity. In the Wistar region of Bellaria, the average age of death is considerably higher than in any other part of the country. Wistar is the only mountainous part of Bellaria. A mountainous terrain makes even such basic activities as walking relatively strenuous; it essentially imposes a physically active lifestyle on people. Clearly, this circumstance explains the long lives of people in Wistar.

Q: Which of the following, if true, most seriously weakens the argument?

(B) The Wistar region is one of Bellaria's least populated regions.

(C) Many people who live in the Wistar region have moved there in middle age or upon retirement.

Here's my reasoning: The stimulus concludes that the the mountainous terrian in Wistar imposes a physically active lifestyle on its residents, and so they live longer.

One way to weaken an argument is to show another cause for an effect. So, (B) looks attractive because the "average age" would have fewer datapoints in Wistar than the country as a whole. This raises the possibility that the average age of death is higher because there's not as many datapoints to smooth out the average.

According to the stimulus, a physically active lifestyle increases longevity. So according to (C), why would it matter when the residents moved there? The stimulus did not say "A lifelong habit of physical activity increases longevity."

Here's GMAT's reasoning: (B), the population density doesn't affect the argument. I think it does, as outlined above...though it's not a knock out punch.

For (C), the GMAT explains that it's correct. It claims the assumption in the argument is that the terrain has shaped residents from birth which accounts for longer life span. If this was an assumption, then (C) would make perfect sense.

So, where my reasoning differs is identifying the underlying assumption: GMAT says author assumed physically active lifestyle would only increase lifespan of people who have been there for life, and I assumed a physically active lifestyle would help increase lifespan of people, regardless of the age they started living that lifestyle (based on stimulus).

Any ideas on reconcilling the two lines of reasoning? Should I just assume that any mention of physically active lifestyles are assumed to only apply to people who have lived that way since birth? That seems fairly narrow (unlikely to encounter again); are there any more general assumptions GMAT seems to make that would apply here? Thanks!