A
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B. To explain how she felt about being discriminated against not only because of her race but also because of her gender.
The function of the infinitive phrase in the sentence <em>To win a marathon is my goal </em>is object of the verb.
It is definitely an object, and it would follow a verb if it were a normal ordering of words in the sentence (My goal is to win a marathon), and since it is not preceded by a preposition, it cannot be object of the preposition, but of a verb.