【单选题】
The sea is the largest unknown part of the worlD、It covers seventy-one percent of the earth. There is still much to be discovered about this vast blanket of water.If the waters of the oceans could be moved away， the sea floor with its wide valleys， irregular mountains and rivers in the sea would be an unbelievable sight. Around the edges of the continents the ocean floor is flat and the water does not become much deeper for about thirty miles. Where there are high young mountains along the coast， this flat part may be much less than thirty miles.But where rivers flow into the sea， the flat area may extent for hundreds of miles.The region near the continents， where the water is not so deep， is the place where the ocean‘s greatest riches in marine life are founD、Below these creatures and plants are the largest known quantities of minerals.At the end of the flat part the sea floor suddenly drops down， forming deep hollows which are shaped like bowls. These huge hollows hold most of the world‘s water.Imagine， if you can， a mountain chain stretching 40，000 miles around the earth. It is hard to believe that this great under-water chain of mountain encircling the earth was discovered only a few years ago.