Which of the following factors makes an epidemiologic study less likely to be valid?

Heyyy! drugs are not good for u. and u should know that! it can injure you or hurt u! drugs itself sounds dangerous! i'm pretty sure u heard many stories about the danger of drugs! even if u are addicted or just want to, pls don't do it! it's harmful even though maybe u don't realize it. until u realize it, it's going to be too late, and i don't want that to happen to u or to anyone. drugs shouldn't be your top priority, just think about it, why do u want to use drugs? u can do so many other things that can relive u, or things u like to do. i might not get what situation u are in, but drugs can't be the thing on your mind all the time, what if the ppl that cares about u gets hurt because, one day they get a call that u are in a hospital; injured or they can have a knock on there door telling them that u past away or something happened. it's not cool and upsetting, obviously i'm not someone who can force u to not do anything u want to do, but pls keep in mind, always think about the people who cares about u, and if u see a child younger than u by a lot, or just younger, u would tell them to not do drugs(maybe) if u do tell them that, u realize it's bad for them, which means u abounds do it either bc its bad. no matter the age, u shouldn't be doing stuff that would hurt u, bottom line is, i can't tell u what to do, u do u, but i'm younger than u, and just want to say pls don't get hurt.

Don't pollution air circulation. wasting pollutionclean in your surroundings