For question 3, why is this not the answer?

"Early twentieth century physicists were puzzled by the existence of black holes."

KERNEL: Physicists were puzzled by black holes.

Subject (Noun): Physicists

Modifiers: Early twentieth century

Predicate (Verb): Were puzzled by

Predicate (Object): Black holes

Modifiers: The existence of

I know its the existence of black holes, but are black holes not a subject? "The existence of black holes" with black holes being the subject, and "the existence of" being the modifier?