Read the paragraph.
(1) Mark loves music, so they want to join the school band next year. (2) Mark wants to play the trombone because he has
a cousin who played it last year. (3) Mark's friend Jill is thinking about joining the band, too, and playing the trumpet
because it is her favorite instrument. (4) The band teacher is happy that Mark and Jill want to join because she
likes teaching new students.
Which sentence has a pronoun-antecedent agreement error?