I am doing practice problems for my exam, and I can't seem to figure this one out.

Let : → , : → . Prove that:

(a) if ∘ : → is onto, then is onto

(b) if ∘ : → is one-to-one, then is one-to-one

I know onto means surjectivity, and one-to-one means injectivity, but the ∘ part is throwing me off. How would I go about proving this? We weren't given examples like this in class, but it is on our review sheet as concepts we should know.

Would it mean ∘ is $A\to C$, or something else...

Thanks for guidance. If you have a proof, that would be helpful as well