If you answered

"By different types of acid resist, artists can get variable tones on their etching plates."

You're wrong! Sorry

In this detail of an etching, fine aquatint
and coarse aquatint produce different tones.
Click on the image to see the whole print.

But you're not entirely wrong because the artist can change the texture of a resist, such as aquatint, for different tones. Continuous or gradations in tone usually can't be done this way, however.

In the detail at left, light gray tones resulted from a very fine aquatint, and heavy, textured tones are from a coarse aquatint.

Do go back and try another choice.

- GM