Two centuries ago, Tufe Peninsula became separated from 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? ["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.", 'There were about as many Turfil sunflowers on Tufe Peninsula two centuries ago as there are on Tufe Island today.', 'There are no types of vegetation on Tufe Island that are known to benefit from dry conditions.', '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.']