In the Modern Theory of Crustal Motion on Earth's Surface,the

In the modern theory of crustal motion on Earth's surface,the process of seafloor spreading is described as the

A) motion of plates away from mid-oceanic ridges and toward continental boundaries.
B) upward motions of some plates and downward motions of others while remaining in fixed positions on Earth.
C) rotation of plates around axes that remain stationary on Earth,causing plate-edge collisions.
D) motion of plates toward mid-oceanic ridges and away from continental boundaries.