I also thought the answer was rather C than D.

Actually there has been no hint either in the passage or question intrinsic factors should be considered over extrinsic moreover even if you consider deforestation
That is future event and should we consider it ?

Can anyone explain what the question is really asking us to answer? the question asks us to Account for Discrepancy? I didnt quite fully understand it

I also had gone for C as my first answer but if we think a little harder, then we can allegedly conclude that the other population(Koluga) was not that reclusive or obscure after all - supported by the fact that has been established in the passage - "the population density of Koluga gorillas is significantly greater than that of Morgania gorillas" - which means that both would have to be accessible and hence comparable, but there's some peculiar characteristic that attacks one of the two factors(ability to capture and thereafter test them) essential for the study.

Here is the OE as below:

The greater the population density the greater the chance a gorilla is infected. Koluga gorillas have a greater population density than Morgania gorillas, therefore one would expect them to be more . Based on captured gorillas, the Morgania gorillas are more likely to be infected.

The paradox can best be resolved by (D). If scientists are far more likely to capture non-infected than infected Kogula gorillas, than that accounts for the difference in results.

(A) would be correct if the passage mentioned that researchers only captured the gorillas in the plains, and uninfected Koluga gorillas are more likely to venture out of their natural habitat.

(B)does not differentiate between the two gorillas so it unlikely to help resolve the discrepancy.

(C) is similar but different. It is not the total number of captured Kogulas that is important. ‘Twice as likely’ is based on rate not total number.

(E) is out of scope.

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