For many years, scientists thought that the ocean floor was flat. During World War II, U. S. Navy ships patrolled the oceans. A captain of one of the ships, Harry Hess, was also a geologist. He used a new device called an echo sounder to map the ocean floor. The echo sounder data showed that the ocean floor had mountains and volcanoes in addition to flat areas. When scientists took samples of the rocks that made up the ocean floor, they discovered something surprising. The rocks closer to the mid-ocean ridge were younger in age than the rocks far away from the mid-ocean ridge. They concluded that this difference in age was another way to support the theory of plate tectonics.

Which best describes how the scientists could explain their observations?

A They thought that sediment washing in from the shore formed the

ocean floor, and continued to build out from the shore.

B They thought that strong ocean waves pushed the younger rock material toward the middle of the ocean.

C They thought that wave erosion at the shore removed the younger rock layers that were on top of the older rock layers.

D They thought the young rock formed at the mid-ocean ridge and pushed the older rock toward the shore.