【正确答案】 C

【答案解析】[解析] 23-26
Not long ago, some of you may have read about a team of mountain-climbing scientists who helped to recalculate the elevation of the highest mountain in the world: Mr. Everest. Of course, the elevation of Mt. Everest was determined many years ago, using traditional surveying methods. But these scientists wanted to make a more precise measurement using a new method that takes advantage of recent advances in technology. It's called the Global Positioning System. The Global Positioning System uses 24 satellites that circle the Earth. Each of these satellites is constantly sending out signals. And each signal contains important information that can be used to determine the longitude, latitude, and elevation of any point on the Earth's surface. Well, in order to use this system to calculate Mt. Everest's elevation, scientists needed to put a special receiver on its summit to receive signals from the satellites. The problem with this was that, in the past, the receivers were much too heavy for climbers to carry. But now these receivers have been reduced to about the size and weight of a hand-held telephone, so climbers were able to take a receiver to the top of Everest and, from there, to access the satellite system signals that would allow them to determine the precise elevation. And it turns out that the famous peak is actually a few feet higher than was previously thought.
Q. 23. What is the talk mainly about?