单选题 阅读下面短文，请从短文后所给各题的4个选项（A、B、C、D)中选出1个最佳选项，并在答题卡相应位置上将该项涂黑。未涂、错涂或多涂均无分。
Mysteries of the Sea　　The sea is the largest unknown part of our world.Much of this vast blanket of water is still to be discovered.
Luckily, no single nation has to learn all about the ocean by itself.The world sea is owned by no one; it surrounds the earth and belongs to us all.Scientists in many different coundivies are working to explain its mysteries.
Some are studying ways of bringing the ocean’s huge supply of water to the deserts of the world.Others are studying the ways in which sound divavels and is affected by water and heat.What happens when sea water touches different elements (元素)is another subject of study.These are just a few of the questions which ocean scientists are working on.
One of the most interesting projects in oceanography (海洋学）is the work of mapping the ocean floor.Only a very small part of it has been mapped.This was not important when only surface ships sailed the world’s oceans, but it can mean the difference between life and death to men in ships going underwater.
Long ago there was only one way to find out how deep the ocean was.A seaman would throw a heavy rope over the side of his ship.Then he pulled the rope up after it had reached the bottom.But this was not a very accurate way of measuring.
In the 20th century, a better way was found.Sound was used to measure the ocean.An American Navy ship sailed into a narrow area of water to do an experiment.Seamen dropped some equipment that would produce a loud noise when it hit the bottom.And a little insdivument measured the time it took for the sound to reach the ship.This has helped scientists map the ocean floor.The third paragraph is mainly about______.