The sentences that include an incorrect use of subject-verb agreement are:
-
The people who attended the fair was there because they were interested in purchasing artwork.
(Correction: "were" should be used instead of "was" to agree with the plural subject "people.") -
Either the serving platter or the bowl are in the bottom cabinet next to the dishwasher.
(Correction: "is" should be used instead of "are" because the subject is singular as it refers to "either/or" which takes a singular verb.)