This answer choice is trying to suggest that people aren't using the best sunscreen available. But the stimulus doesn't say anything about price differences among sunscreens, or whether the most expensive sunscreens are more effective. The argument is about sunscreen use in general and its connection to skin cancer rates. So this answer choice doesn't apply to the stimulus.

If this is true, then most cases of skin cancer affect older people as a result of sunburns from the beginning of their lifetimes. In other words, the skin cancer rates observed during the researchers' study were likely mostly caused by sun exposure that happened well before the 25-year period began. In other words, the increased rates of skin cancer during those 25 years have little to do with whether or not people were using sunscreen during that time. The effects of the increased rates of sunscreen use are likely to show up much further down the line. This strongly weakens the link between the evidence in the stimulus and the author's conclusion.

This doesn't weaken the author's argument. It's possible that the dermatologists' research was inaccurate. We need something that tells us why the positive correlation between skin cancer and increased sunscreen use doesn't actually mean that sunscreen is ineffective.

This answer choice only tells us that one particular group of people is likely to stay out of the sun. That doesn't tell us anything about sunscreen use in general and increased rates of skin cancer. This doesn't weaken the argument.

Whatever people believe about themselves, we're interested in what actually happens: does using the sunscreen reduce their risk of skin cancer or not? This answer choice doesn't give us that information, and doesn't weaken the author’s claim.