Answer: b
Explanation: Heat is produced through the radioactive decay of elements in the inner core. This heats the lower mantle, causing the magma to become less dense and rise. Once it reaches the upper mantle it cools, becomes more dense and sinks. Tectonic plates are able to move because Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly-formed crust cools and moves away from the ridge). At subduction zones the relatively cold, dense oceanic crust sinks down into the mantle over the downward convecting limb of a mantle cell. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.