How to nicely reject a guy?

I have a feeling this guy is going to ask for my number soon, and I have some interest in him, but I think I would decline him because I'm leaving for college in a year and I'm super busy as is. this guy is basically a guy I see at the store, so I don't really know him but I just get the feeling he's going to ask. what should I do? and if I decline how do I not feel bad?
How to nicely reject a guy?
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