Two sentences correct the vague pronoun problem in the sentence, and they begin with "Until Joe became too tired," and "Until his brother became too tired."
A pronoun is a word that refers to another word in the sentence. The word the pronoun refers to is called an antecedent. When we have two or more possible antecedents for one pronoun, we have what is called a vague pronoun.
Let's take a look at the sentence, "Joe kept pace with his brother as they were running around the track unit he became too tired and had to stop." The problem here is that the pronoun "he" can refer to either Joe and his brother. Thus, it is a vague pronoun.
In order to correct that problem, there are two possibilities:
Learn more about vague pronouns here:
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