Complexity classes that are closed under subtraction

Question

Are NP or P closed under subtraction? Im having a hard time deciding whether they are or aren't. Question was edited

Original question: Im having some hard time figuring out what languages are closed under subtraction. Say you have 2 languages A, B ∈ NP. Is A\B ∈ NP? what about P?

Commenters: My original question was extremely not accurate so i rephrased :)

Thanks!