In the latter part of the 19th century the British ship HMS Challenger was specially equipped for a scientific expedition. It made significant studies of the ocean floor. Later theories dealing with continental drift and sea floor spreading were based on which of the two discoveries made by the HMS Challenger expedition?

Oceanic ridges and

Deep sea trenches.

Explanation:

DEEP SEA TRENCHES are places where the speeding sea floor runs out of room and the ocean plates are subducted, form where crustal plates plunge back into the mantle.

OCEAN RIDGES are spreading centers; that is, they are divergent plate boundaries where magma wells up from below. Where new crust is being formed.