We know more about the surface of the planet Mars than we know about what is at the bottom of Earth's oceans. Until recently, scientists had mapped only about 20 percent of the sea floor. But our knowledge of the deep seas is changing because of information from satellites. Scientists have produced a new map that provides a detailed picture of the oceans. The map is expected to help oceanographers, industry and governments. The new map is twice as detailed as the map made 20 years ago. David Sandwell is a geophysics professor at the Scripps Institution of Oceanography at the University of California. He and other scientists worked on the mapping project. He says they turned to space to look deeper into the ocean. He spoke to VOA on SKYPE. "The way we're doing that is to use a satellite altimeter , a radar to map the topography  of the ocean surface. Now that seems sort of strange that you'd map the topography of the ocean surface when you really want to get at the sea floor. But, the ocean surface topography has these bumps and dips due to gravitational effects that mimic what's on the sea floor." The scientists collected and studied information from two satellites. Some of the data came from the European Space Agency's Cryo-2 satellite. It was placed in Earth's orbit to watch sea ice. The other satellite is the American space agency's Jason-1. It is studying the surface of the oceans. Scientists combined the satellite data with images gathered by sonar equipment on ships. Sonar works by sending sound waves through the water. When the sound waves hit an object, its presence is confirmed. "That enables us to look at smaller-scale features and also features that are buried by sediments in the ocean basins." The new map shows the sea floor as it has never been seen before. It shows thousands of underwater mountains and places where continents pulled apart. It shows where earthquakes were active many years ago. They all are buried deep underneath the ocean floor. In one place, three mountain ridges join at the same area. Huge tectonic   plates can be seen clearly there. Scientists collected and studied information by   _  .
the European Space Agency's Cryo-2 satellite the American space agency's Jason-1 sonar equipment on ships the Cryo-2 and the Jason-1
Question: We know more about the surface of the planet Mars than we know about what is at the bottom of Earth's oceans. Until recently, scientists had mapped only about 20 percent of the sea floor. But our knowledge of the deep seas is changing because of information from satellites. Scientists have produced a new map that provides a detailed picture of the oceans. The map is expected to help oceanographers, industry and governments. The new map is twice as detailed as the map made 20 years ago. David Sandwell is a geophysics professor at the Scripps Institution of Oceanography at the University of California. He and other scientists worked on the mapping project. He says they turned to space to look deeper into the ocean. He spoke to VOA on SKYPE. "The way we're doing that is to use a satellite altimeter , a radar to map the topography  of the ocean surface. Now that seems sort of strange that you'd map the topography of the ocean surface when you really want to get at the sea floor. But, the ocean surface topography has these bumps and dips due to gravitational effects that mimic what's on the sea floor." The scientists collected and studied information from two satellites. Some of the data came from the European Space Agency's Cryo-2 satellite. It was placed in Earth's orbit to watch sea ice. The other satellite is the American space agency's Jason-1. It is studying the surface of the oceans. Scientists combined the satellite data with images gathered by sonar equipment on ships. Sonar works by sending sound waves through the water. When the sound waves hit an object, its presence is confirmed. "That enables us to look at smaller-scale features and also features that are buried by sediments in the ocean basins." The new map shows the sea floor as it has never been seen before. It shows thousands of underwater mountains and places where continents pulled apart. It shows where earthquakes were active many years ago. They all are buried deep underneath the ocean floor. In one place, three mountain ridges join at the same area. Huge tectonic   plates can be seen clearly there. Scientists collected and studied information by   _  .
A. the European Space Agency's Cryo-2 satellite
B. the American space agency's Jason-1
C. sonar equipment on ships
D. the Cryo-2 and the Jason-1
Answer: