10. well into the twentieth century, many scientists believed the ocean floor was a. rocky. b. round. c. flat. d. sandy.

In the 20th century, the scientist believed that the floor of the ocean floor is flat. But later on discovered that their theory is wrong. The ocean floor is not flat and featureless. One technology helped the scientist to study in advance and in details the real feature of the ocean floor. They used sound waves to know the distance of the water from floor level to the top. They also discovered that ranges of mountains lies under the ocean. Sonar was one of the technologies they used to study the sound waves emitted by the ocean. Hope i was helpful :)

D. is the answer hope that you