The major areas of medicine in which lasers are effective is in the cutting and closing of blood vessels, and in the destruction of tumors.

The correct answer here is "are the cutting and closing of blood vessels, and the destruction" of tumors.

the Princeton guide says: "As you know, some nouns require an "-ing" ending. "Cutting" is one, but the noun form of "destroy" is not " the destroying" its "the desctruction"

But cutting adn closing here are verbs. Why does princeton talk about nouns? Also to be parallel, the answer should be "and the destroying of tumors", shouldnt it?