When we're looking for an ASSUMPTION on which the argument DEPENDS, we want to ask ourselves, "does this thing HAVE to be true for the conclusion to hold?"

Many students mix up STRENGTHEN and ASSUMPTION questions. For strengthen, we simply ask "does this help the conclusion? Does it make the conclusion more likely to be true?" But it's not necessarily something that has to be true.

Here's the structure of this argument:

Premises:
- Tufe Peninsula became separated form the mainland, isolating on the newly formed Tufe Island a population of Turfil sunflowers
- Tufe Island sunflowers are 40 centimeters shorter than Turfil sunflowers found on the mainland
- Tufe Island is significantly drier than Tufe Peninsula was

Conclusion:
The current average height of Tufe's Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe's environmental conditions.

Sometimes the best way to find out what the argument is missing is to ask, "is there any way for the conclusion NOT to be true, while keeping the premises true?"

We know that the island and the mainland have different environments now, and that the island has shorter sunflowers. We even know that the island is drier than the peninsula used to be. But could there be some way for the height difference in sunflowers NOT to be caused by the island's changes? What about the mainland's changes? We don't know about the height of sunflowers 200 years age. We're assuming the island flowers shrank, but maybe the mainland ones grew!

A. There are no types of vegetation on Tufe Island that are known to benefit from dry conditions.
Other types of vegetation are irrelevant to our conclusion about sunflowers.

B. There were about as many Turfil sunflowers on Tufe Peninsula two centuries ago as there are on Tufe Island today.
NUMBER of sunflowers = irrelevant to conclusion about HEIGHT of sunflowers.

C. 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.
Bingo! Try negating this one - if the mainland's environment DID change, then our conclusion falls apart.

D. 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.
This one strengthens - it gives more reason to believe that changes on the island affected the sunflower height. But does it HAVE to be true? Nope! Other changes besides the soil could still account for it. If we negate it, it doesn't destroy the argument.

E. 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.
Irrelevant. The height difference is the only difference that our conclusion deals with, but it wouldn't have to be the only difference between the two species.

Hope that helps!