Question

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.

Explanation:

Deep - ocean trenches are formed at subduction zones. In a subduction zone, the denser oceanic plate slides (subducts) under the less - dense continental or oceanic plate. Crust is formed at mid - ocean ridges (not trenches). Mountains are formed in various ways like at convergent boundaries (but not specifically the main event at trenches). Seafloor spreading occurs at mid - ocean ridges.

Answer:

D. Denser plates slide under lighter plates