Answer:
a) Gravity pulling the older and denser lithosphere downward.
Explanation:
Earth's crust is made of plates which float on mantle . Plates are of two types
1 ) oceanic plates which are older and heavier 2 ) continental plates which are lighter . Continents are situated on continental plates. Due to relative motion , oceanic plate, because of its heavy weight, goes under lighter continental plate . This is called subduction.
