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 countries 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 travels 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 oceangraphy（海洋学） is the work of mapping the occean 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 instrument measued 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 _____.
A、the elements in the sea
B、the weather on the sea
C、the studies of the sea
D、the sound under the sea
【正确答案】：C
【题目解析】：题干大意：第三段主要谈论什么。定位到文中的第三段，中心句第三段的第一、第二句，“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 travels and is affected by water and heat.”，一些人正在研究如何将海洋大量的水供应到世界上的沙漠。另一些人正在研究声音传播的方式，并受水和热的影响。由此可知，第三段主要讲述的是不同科学家对海洋所做的研究工作。故选C。
