For the problem about confounding, could a confounding variable be that some people might eat more junk food and thus will have a higher number of cavities than other people?
You have to relate it to both variables. You have to explain yourself more than that.
You could say people who eat fewer apples might also eat more junk food since they don’t eat a lot of fruits. Eating more junk food can increase the number of cavities. Therefore causation cannot be established.
You can say something like that!