Which Observation Constituted Evidence for the Process of Opponent Color

Which observation constituted evidence for the process of opponent color representation?

A) Colors observed in the afterimages are the other member of the opponent color pairs.
B) Colors often appear to be mixtures of two opponent colors but never appear to be mixtures of two nonopponent colors.
C) People sort a stack of differently colored cards into three piles-red, green, and blue.
D) Ewald Hering discovered a fourth cone type (for yellow wavelength) using retinal densitometry.