Video Explanation

Text Explanation

The credited answer is (D). If this is true, then it may be that, as expected from the correlation, Koluga gorillas actually would be infected at a higher rate, but because the infected Koluga gorillas are aggressive and difficult to test, fewer of them actually would be tested, and most Koluga gorillas tested would wind up being the un-infected ones; this would lower the rate of ones that appear to be infected. This would explain the discrepancy.

According to the correlation model, more Koluga gorillas should be infected. If some move down to the plains for segments of the year, then during those times they might experience lower population densities, but then rain comes, and they go back up the mountain to rejoin the high density conditions, where they easily could be infected. This would not affect the overall infection rate. Choice (A) is incorrect.

If dormant strains of the virus are difficult to detect, presumably it would be difficult to detect in both Koluga gorillas and Morgania gorillas, and doesn’t explain a difference in rate between the two. Choice (B) is incorrect.

The argument is about percentages, about rates. We are trying to explain a discrepancy in expected rates. Actual numbers of one kind vs. the other kind of gorilla are irrelevant. Choice (C) is irrelevant and incorrect.

As long as researchers can tell the difference between the two kinds of gorillas, which doesn't seem to be an issue, then coloring similarities and species classifications are irrelevant. Choice (E) is irrelevant and incorrect.

Frequently Asked Questions (FAQ)

Why can't Choice C explain the discrepancy in the data? Couldn't a larger sample of gorillas include more infected ones? 

It's definitely true that the a sample size for a study like the one described in the question can skewer the results of that study. However, in this case, we'd actually expect the sample of Koluga gorillas to display abnormalities instead of the sample of Morgania gorillas, since the sample of Koluga gorillas would be smaller. After all, the larger a sample is, the more we can trust the data that it represents. So it looks like Choice C won't do a whole lot to explain the specific discrepancy that is mentioned in the question. 

Choice D does a much better job of this because it describes an important behavioral difference between infected and non-infected Koluga gorillas. If infected Koluga gorillas act in a way that inhibits their sampling (being aggressive), that will have a strong impact on the results of researchers' studies, since the infected gorillas actually won't make it into the sample in the first place.