Which of the following is the antecedent of the pronoun theirs in the sentence below?

Rivers stopped flowing along the banks so that they could hear Orpheus, whose music was more harmonious than theirs.
Responses

rivers
rivers - not selected, this is the correct answer

banks
banks - no response given

music

music - not selected

why is rivers the correct answer?

The antecedent of a pronoun is the noun or phrase that the pronoun refers to or replaces. In this sentence, "theirs" is referring back to something that was previously mentioned. The sentence mentions "Orpheus, whose music was more harmonious than theirs." The pronoun "theirs" is referring back to Orpheus's music, indicating that the antecedent is something possessed by Orpheus. Since "music" is the possession of Orpheus and is being compared with the music of the rivers, "rivers" is the correct antecedent for the pronoun "theirs."