An ocean trench .
a. marks the upwelling of mantle material.
b. is the site of submarine volcanoes.
c. occurs where a plate in moving down.
d. started breakup to the single southern continent.
.

C. Occurs where a plate is moving down.

Explanation:

Ocean trenches are steep depressions in the deepest parts of the ocean where old ocean crust from one tectonic plate is pushed beneath another plate, raising mountains, causing earthquakes, and forming volcanoes on the seafloor and on land.

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