What Important Physical and Chemical Process Occurs to Subducted Seafloor

Multiple Choice

What important physical and chemical process occurs to subducted seafloor as it reaches a depth of 100 kilometres depth in the upper mantle?

A) It experiences nearly a doubling in its density as the subducted basalt recrystallizes to peridotite.
B) The subducted seafloor entirely melts generating a line of explosive volcanoes.
C) Heat from the surrounding mantle drives off water and other volatiles from the slab, which escape upwards, lowering the melting point of the overlying asthenospheric mantle.
D) The cooling of the ocean crust from the cold, deep ocean water of the trenches causes the subducted crust to freeze the mantle, cracking and freezing it to generate the largest earthquakes.