the rsults suggest that the rats behaved as though they assumed that if they became sick this must have been caused by something they ate, but that exposure tonoises and lights were not the sorts of thing that could cause sickness.

위 문장에서 assumed의 목적어로 that if ~와 that exposure ~sickness가 but을 기준으로 병렬구조를 이루고 있다고 합니다. 그럼, 밑줄 친 were의 주어는 exposure가 되어야 하는거 아닌가 해서요. 그럼 왜 was가 아닌 were 일까요?