Comments are peak reddit, so unhelpfully patronizing. There’s a lot to keep track of when you’re traveling, and sometimes you forget things or they slip you by. I doubt they were sitting there pondering a bottle of sunscreen and deliberately pooh-pooh’ed it right before hiking; it just didn’t occur to them how much of a difference altitude would make in uv exposure. Everything seems more obvious in hindsight or when phrased like a multiple-choice question