Do you know why a slice of apple turns brown when you leave

Do you know why a slice of apple turns brown when you leave it in the open air?Now you can find out the answer with this fun and simple a___1____.
First,you should take an apple and c__2____it into at least two pieces.Take one slice and put it in a bowl of water——just e___3___water to cover the slice of apple.Then leave the second slice out in the open.
After a few minutes,you can see that the slice of apple in the water does not turn brown i___4___.The water covers it and keeps it s___5____ from the effects of oxygen in the air.
The other slice of apple is in the open air,so it turns brown much q____6___than the first one.