The Term "Quadrature" in a Quadrature Detector Refers To

The term "quadrature" in a quadrature detector refers to:

A) their use in producing four-channel stereo detection
B) the fact that it was the fourth FM detector design to gain popularity
C) the fact that the two input signals to the exclusive-or gate are 90 degrees out of phase with one another
D) the fact that a four input gate is part of the detector stage