Subduction occurs as a result of:
a) Gravity pulling the older and denser lithosphere downward.
b) Horizontal plate accommodation.
c) Upwelling of hot mantle material along the trench.
d) Lubrication from the generation of andesitic magma.

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.  

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explanation: