the results suggest that the rats (behave/behaved) as though they assumed that if they became sick this must (be caused/have been caused) by something they ate, but that exposure to noises and lights (was/were) not the sorts of thing that could cause sickness.

밑줄친 문장에서 왜 답이 were가 되는지 궁금해요. that이하의 문장안에서 주어는 exposure단수 인것 같은데요.
답변 부탁드립니다.