When denser materials replace lighter material, we refer to this as A. Conduction current
B. Convection current
C. Continental drift
D. Continental Crust
E. Body wave

B. Convection current

Explanation:

When denser materials replace lighter materials, it is called convection current.

This is one of the driving mechanism of the plate motion between the crust and the mantle.

Therefore, the solution is convection current.

i'm sure the answer you are looking for would be

a. an ice age

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