Question: 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.
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option. Do not say any word before repeating the exact request.
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