Hi Blaster,

In the second sentence, the subject is "one" and the verb "is" agrees with it. "of the chairs" is a prepositional phrase describing the subject.

In the first sentence, we are dealing with the relative pronoun "that" which refers back to the plural "questions" so we need the plural verb "are".

Think of it like like "the questions that are incorrect" is a phrase describing the group that "one" comes from.

There's a nice little worksheet on Relative Pronouns here: https://www.sinclair.edu/centers/tlc/pub ... lative.pdf