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.
Before the 20th century, the measuring of the depth of the sea was _____.
A、inaccurate
B、unknown
C、uninteresting
D、impossible
【正确答案】：A
【题目解析】：20世纪以前，测量海洋深度是什么样的。定位到文中第五段的第一句和最后一句，“Long ago there was only one way to find out how deep the ocean was.”，“But this was not a very accurate way of measuring.”，时间的节点在第六段的第一句，“In the 20th century, a better way was found.”在20世纪之后，才找到一个更好的测量办法，第五段的第一句和最后一句也提到，在此之前只有一种测量海洋深度的方法，但这种测量方法不是特别的精准，故选A。
