A、A thrilling exploration in the desert. B、How GPS saved the survivors. C、What is GPS and how it works. D、The danger of space ex

For three groups of explorers formed by college students and young white collars, it started out as a wonderful trip. They were enjoying themselves exploring the Inner Mongolian desert. [12] But when the sandstorm came, the trip became a disaster.
　　The drama unfolded during the May Day holiday. One girl did not make it before the students were rescued. The group was eventually found with the help of a GPS receiver. GPS, or global positioning system, is a satellite-based navigation system made up of a network of 24 satellites. It was originally created for military use by the US in 1980s but is now available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. And the [14] service is free.
　　GPS satellites circle the earth twice a day and transmit signals and information to the earth. GPS receivers take this information and calculate the user’s exact location. A GPS receiver must be locked on to the signals of at least three satellites to calculate the latitude and longitude of its location. With four or more satellites in view, the receiver can determine the user’s altitude. [15] Once the user’s position has been determined, the GPS receiver can calculate other information, such as speed, direction, distance to destination, and a whole lot more.
Questions 12 to 15 are based on the passage you have just heard.
12.What is true about the trip made by the young people?
13.What is the main topic of the passage?
14.Which of the following is true about the GPS?
15.What must be determined first before the GPS receiver can calculate other information?