Why do we give teachers apples?

Don't you just love it when I post a random fact that doesn't really have an answer? I was asking myself, why do we give apples to teachers? Seems like a straightforward enough question. Here's what I found though:

This stems from the saying "An apple a day keeps the doctor away".

The apple is the symbol of knowledge.

Back in the day, school had to be paid for. So poor farmers would bring their crops into the teachers. Of course, teachers had no way to keep fresh fruit for long periods of time, so the student would bring in an apple a day as payment.

So which one do you think it is? I think that the third one sounds the most plausible, but this may just be one of those things that doesn't have a straightforward answer.