A、To indicate how climbers communicated. B、To show that climbers enjoyed many comforts. C、To show that modem telephones can work

Not long ago, some of you may have read about the team of mountain-climbing scientists who helped to recalculate the elevation of the highest mountain in the world, Mount Everest. Of course the elevation of Mount 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 the satellites is constantly sending out signals, and each signal contains important information that can be used to determine the longitude, latitude and elevation at any point on the earth’s surface. Well, in order to use this system to calculate Mount Everest’s elevation, scientists need 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 the receiver to the top of the 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.
15. What is the talk mainly about?
16. Why does the speaker mention a hand-held telephone?
17. According to the talk, what probably contributed most to the success of the new study of Mount Everest?
18. In the study described, how were satellites’ signals used?