Let S, T, and U be nonempty sets, and let f: S -> T and g: T -> U be functions such that the function g ∘ f : S -> U is one-to-one (injective). Which of the following must be true?
(A)f is one-to-one. (B)f is onto. (C)g is one-to-one. (D)g is onto.
A