I'm kind of confused about the use of passive voice in question 3, but I'm not sure if I'm just being too nitty-gritty about grammar. Written in active voice, it would say "the existence of black holes puzzled early twentieth century physicists," so the subject would be "existence" (modified by "of black holes") and the predicate-verb would be "puzzled" (modified by "Early twentieth century physicists"). I thought changing between active and passive voice does not actually change what the subject vs object would be, so even though it's written in passive voice the subject would still be the black holes. Am I overthinking this?