decision problem – show that NP is a subset of decidable languages

More specifically the problem says: “Let us call the set of decidable languages D. Show that NP ⊆ D”

My problem is that I always assumed that NP is decidable, but to prove it, I never thought it
Somebody have any idea about how do it? or start resolving it?

I think I can prove it saying something like the NP problems effectively end in a polynomial time (although not deterministic) so there must be a MT-ND that accepts it and always ends, I suposse