Sea-floor spreading is a valuable theory as it relates to plate tectonics because
a. it replaced plate tectonics as the main theory explaining geologic processes.
b. it disproved the theory of continental drift by providing evidence that tectonic plates cannot move vast distances.
c. it was combined with the theory of continental drift to form one unified explanation.
d. it showed how the spreading of the ocean floor can cause earthquakes, which explained continental drift.