What is a gerund, and how does it function in a sentence

it Language Arts

I believe it's C: A gerund is a noun that functions as the subject of a sentence.

Explanation:

Elizabeth Here!!^^ I would say it C: A Gerund that function in/on the subject of a sentence.

(wasn't fully sure, but i think i done with before since last year. yet, also im not sure if correct. sorry if it incorrect!^^)

changeable lenses of different focal lengths allow light to enter the camera, which then forms the image on light-sensitive film" is the one among the following statements about working principle behind a basic still camera is true.