Question

what happens to the oceanic crust that subducts under the oceanic crust?
the material stays at the surface and adds to the crust
the material sinks, melts and becomes molten material due to intense heat and pressure
the material remains solid and joins the oceanic crust
nothing

Explanation:

When oceanic crust subducts under other oceanic crust, the intense heat and pressure in the mantle cause the subducting material to sink, melt, and turn into molten material. It doesn't stay at the surface to add to the crust, nor does it remain solid and join the oceanic crust, and clearly, something does happen.

Answer:

The material sinks, melts and becomes molten material due to intense heat and pressure