Explanation

Reading the question: we can see that this question looks like a lot of work. First, it has given us a difficult-seeming puzzle: this country is has a low cancer rate, for non-genetic reasons, but people who move there have a higher cancer rate. Secondly, we have a "NOT" question. Questions with "NOT," like those with "EXCEPT," tend to be labor intensive, since it's relatively slow to go through four good explanations and one bad one. The key technique here is to use a very basic filter. In "NOT" and "EXCEPT" questions--and all questions that are involved--you're often basic served by a basic filter.

Applying the filter: carefully asking this simple question, we can judge that (D) is the least explanatory of the cancer rate and is therefore the answer. In (D), even if the exact "factor" hasn't been determined, it doesn't matter, because the people who move to Orctania "attempt to follow the practices of the inhabitants." So, we don't know whether it's being pollution-free and stress-free and drug free and so on, but the people are doing all of them, so (D) doesn't help to clear up the mystery.Everything else is a good explanation. Choices (B) and (E) both point out that moving to the country does not automatically equate with enjoying the benefits of the environment or with adopting the behaviors. (C) points out that the population moving to the country might be doing so because they are unusually cancer-prone to begin with, which would explain why they would have a higher rate of cancer even after reaping benefits. Choice (A) points out the possibility that some forms of cancer aren't helped by living in Orctania--those kinds depend only on genes, and, critically, it's possible that the people who move to the country have genes more prone to those forms of cancer, somewhat like the case of (C).

Logical proof: finally, we can use a test of (D) that is similar to the negation test: say that it has been established which factor was responsible. That's not automatically an explanation, because, for all we know, the people who move to Orctania are perfectly recreating/copying that factor. Since the situation is unchanged given (D) or its negation, (D) is unimportant to the situation. The correct answer is (D).