Can any professional teacher finally help me with this question? This question is a part of a Cloze passage from MA exam Nealson and Hastings knew the light resulted from chemical reactions........ by the enzyme luciferase. 1)catalyzed 2)were catalyzed 3)was catalyzed 4)when catalyzed The answer key says (1) , because the sentence is in the past form and not in passive . But I can't understand how it knows!. When I want to answer this question I make it complete like this " Nealson and Hastings knew the light which was resulted from chemical reactions was catalyzed by the enzyme luciferase." I mean the adjective clause when begins is in the passive form so how the rest of the sentence shouldn't be in the passive form. The only time I can persuade my self that the choice 1 is correct is in this form . "Which was resulted from chemical reactions that was catalyzed". I really have problem with this sentence and no one really break it down for me .please if you can tell me how can I select the right answer in these situations.