Mark and Jacob Are Driving Home During a College Break

Mark and Jacob are driving home during a college break. Mark is in the passenger seat. Mark is bored and gazes toward an empty field. He notices that distant hilltops seem to be moving slowly in the same direction in which their car is moving, whereas mile markers on the side of the highway seem to moving swiftly past them in the opposite direction. The difference in the apparent speed and direction of the objects' motion serves as a depth cue known as:

A) motion disparity.
B) motion parallax.
C) motion perspective.
D) motion gradient.