Which statement describes one event that happens at deep-ocean trenches? Crust is formed
Mountains are formed
Seafloor spreading occurs
Denser plates slide under lighter plates

The statement that describes the event that happens at deep-ocean trenches is ( D ) ; Denser plates slide under lighter plates

Deep ocean trenches are created by subduction ( the converging of two or more tectonic plates ). whereby the more denser plate is pushed under the lighter plate into the earths' mantle.

The denser plate pushed into the earths' mantle will cause the seafloor and outermost part of the earth to bend into a steep forming the  deep-ocean trenches.

Hence we can conclude that the statement that describes the event that happens at deep-ocean trenches is Denser plates slide under lighter plates.

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the answer is:

carbon dioxide