I got the answer correct, but I was wondering if anyone else came to the conclusion that D was incorrect through a different way than as provided. For me, I read D as saying those who were vulnerable spent less time in the sun, meaning they wouldn't be a relevant party to the "experiment," to begin with. Because they are already staying out of the sun, there is no way to assess what impact sunscreen or the sun has on them. But the explanation seems to imply that they are part of the "experiment." Is this train of thought technically wrong?

I find that when completing Weaken questions, process of elimination is usually adequate for finding the correct answer choice. But, of course, it is different for everyone, and you should use the strategy you feel most comfortable with.