GWD2-40

Q40:

Two centuries ago, Tufe
                Peninsula became separated form the mainland, isolating on the newly formed Tufe
                    Island a population of Turfil sunflowers.  This population’s descendants grow to be, on average, 40 centimeters shorter than Turfil sunflowers found on the mainland.  Tufe
                Island is significantly drier than Tufe
                    Peninsula was.  So the current average height of Tufe’s Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe’s environmental conditions.

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

1. There are no types of vegetation on Tufe
                           Island that are known to benefit from dry conditions.
2. There were about as many Turfil sunflowers on Tufe
                       Peninsula two centuries ago as there are on Tufe
                           Island today.
3. The mainland’s environment has not changed in ways that have resulted in Turfil sunflowers on the mainland growing to be 40 centimeters taller than they did two centuries ago.
4. The soil on Tufe
                           Island, unlike that on the mainland, lacks important nutrients that help Turfil sunflowers survive and grow tall in a dry environment.
5. The 40-centimeter height difference between the Turfil sunflowers on Tufe
                           Island and those on the mainland is the only difference between the two populations.

为什么选C啊，我觉得C不是assumption，而是weaken啊~~

我把这张天顶上来，是因为没找到其它相关的讨论了

我也和lz一样觉得是在问weaken