Options are:
a). people can drive along a desert easily.;
b). All GPS devices have electronic maps;
c). Ground stations control the satellites and make sure they are working well.;
d). Everyone will use the GPS for free.;
Given those answer options, answer the question: Read the following article and answer the question. Article: Imagine the situation. You are driving along a desert or on a mountain. You have no idea where you are. You passed the last house two hours ago. Then your car breaks down. It is night and it is cold. You have no mobile phone. What do you do? Well, next time take a GPS with you. This invention may be able to help you. It is a device which uses satellites to find the user's position. It can find your position in 20 meters. A GPS can't start your car, but at least you will know where you are. GPS, which means Global Positioning System, is a small radio receiver. It looks like a mobile phone. You can hold it in your hand, or put it in your pocket. It is sometimes put into a watch or a telephone. We also find GPS devices in cars, planes, or boats. Some of these devices have electronic maps, so you know where you are. For example, in a city they can tell you the name of the street. There are three parts to the Global Positioning System. The first part is the receiver. You can hold it in your hand, or have it fixed into your car, plane, etc. The second part is a group of satellites orbiting the earth. The receiver contacts at least four of the satellites and calculates its position. The third part of the system is a network of ground stations. They are all over the world. They control the satellites and make sure they are working well. Some people think that in the future the GPS will be as common as the mobile. They are becoming cheaper and more and more accurate . There are also many new uses for the GPS. Perhaps they will become like watches. Everyone will have one and you will never be lost again. Question: . From the passage we know that _ . Answer:
A:
c).