The pronoun that replaces the underlined word correctly is "them". The correct option is b.
Pronouns are the words that are come in the place of nouns, when nouns are repeat in sentence more than one time. The pronouns make the sentence look more meaningful.
The pronoun is he, she, they, it, etc.
In the sentence, the underlined word is runners.
Thus, the correct option is b, them.
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