In subduction zones, dense pieces of oceanic crust are pulled downward toward the mantle. which of the following forces drives this process?

Force of gravity drives to the process of subduction zones where dense pieces of oceanic crust are pulled downward toward the mantle.
We can term subduction as the process by which oceanic crust sinks into the mantle when it collides with continental crust. Subduction destroys oceanic crust by forcing it to the mantle where it melts.
Oceanic crust is said to be subducted because they contain more mafic minerals and again it has convergent boundaries. When there is a divergent between continental crust one oceanic crust sinks into the mantle.

C. gemstones they are the only non metallic mineral from the list.