Are formed when one crustal plate is forced under another.
a. rift zones
c. mid-ocean ridges
b. trenches
d. continental shelves

The correct answer to the question above is (b.) trenches. Trenches are formed when one crustal plate is forced under another. It often occurs at an underwater subduction zone. It is generally deeper than it is wide. 

Trenches are formed when one crustal plate is forced under another.

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Answer steam fog;