Question

select the sentence that best supports the idea that sonar helped scientists learn new things about the make - up of the ocean floor. before the 1900s, people were not certain what the ocean floor looked like. they could only measure the depth in one spot at a time, by dropping a weighted line into the water. by the 1920s, people were using sonar, a technology that measures how long a sound wave takes to bounce back from the ocean floor. scientists discovered deep - sea trenches and mid - ocean ridges.

Explanation:

The second - sentence describes how sonar was used in the 1920s and the new discoveries (deep - sea trenches and mid - ocean ridges) scientists made about the ocean floor with its help.

Answer:

By the 1920s, people were using sonar, a technology that measures how long a sound wave takes to bounce back from the ocean floor. Scientists discovered deep - sea trenches and mid - ocean ridges.