Subtractive - because they remove material from the object being ground.

Printing is a subtractive process; all the colors combine to form black.

Subtractive. He chiseled marble rocks.

Is this a trick homework question? I don't believe there is a 100% truthful answer because a subtractive colour is made up of two primaries, and a primary colour is not subtractive any more than a subtractive colour is primary ("...primary subtractive colour..."). Neither can there be a "main" subtractive colour if we are to consider your use of the word "primary" in your question to mean that.

The complementary colour to red is cyan, which is made up of blue and green in equal amounts.

divisive