The correct answers are A. They say strange things happen during a full moon; B. Phil visited Uncle James shortly after his vacation; and D. After her accident, Julie helped Lian fix her broken-down bike.
Explanation:
In grammar, a vague pronoun occurs when the relationship between a noun or antecedent and a pronoun that refers back to it is not clear, which leads the reader or listener wondering to whom the pronoun refers to. This grammatical mistake occurs in the case of "They say strange things happen during a full moon" because in this case the pronoun "they" is not specified and therefore is vague; also, this happens in "Phil visited Uncle James shortly after his vacation" because it is not clear whether "his vacation" refer to Phil or on the opposite to Uncle James which creates a vague construction. Finally, there is vague pronoun in "After her accident, Julie helped Lian fix her broken-down bike" because similar to the previous sentence "her bike" can belong to Julio or Lian. Thus the sentences that need to be revised to correct vague pronouns are A, B, and D.
