Galapagos Tortoises vs Mainland Tortoises

The Galapagos Islands formed at least 8 million years ago and a tortoise population from the South American mainland was estimated to have found its way to the island at least 5,000 years ago. A small isolated population of Galapagos Tortoises emerged. Based on a sample, this population's descendants can grow to be on average, 25 kilograms heavier than similar tortoises found on the South American mainland. The Galapagos Islands are drier than the South American mainland. Thus, the current average weight of Galapagos Tortoises is undoubtedly at least partially attributable to changes associated with the Galapagos' environmental conditions.

Which of the following is an assumption on which the argument depends?

(A) There are no types of vegetation on the Galapagos Islands that are known to benefit from dry conditions.
(B) There were about as many tortoises on the mainland five thousand years ago as there are on the Galapagos Islands today.
(C) The mainland's environment has not changed in ways that have resulted in tortoises on the mainland growing to be 25 kilograms lighter than they were five thousand ago.
(D) The soil on the South American mainland, unlike that on the islands, lacks important nutrients that help tortoises survive and grow heavy in a dry environment.
(E) The 25-kilogram weight difference between the tortoises on the Galapagos Islands and those on the mainland is the only difference between the two populations.



Sourced from GMAT Pill Platform CR Question