I am given to identify if each parts are "injective" or "not necessarily injective", but how do I identify infectivity when the only given conditions whether they are bijective? What is the relationship? Thank you!

Let S, T be nonempty sets. Suppose f : S -> S is an injective function and that g : T - S and h : S - T are functions such that go h is bijective. For each of the following functions, determine whether it is injective. (a) hof [ Select ] (b) fog [ Select ] (c)hog injective