17．
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（裝置） which uses satellites （衛(wèi)星） to find the user's position（位置）．It can find your position to within 20metres．A GPS cannot 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 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（計算） 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（精確的）．There are also new uses for the GPS．Perhaps they will become like watches．Everyone will have one and you will never be lost again．
71．According to the passage，with the help of the GPS，peopleA．
A．can't be lost in a new city
B．can't find their way in different countries
C．can learn about the culture of an unknown place
D．can spend the least time getting to another place
72．We can learn from the passage thatA．
A．there are three parts to the GPS
B．a GPS can't be put into a watch
C．a GPS can help you start your car
D．the GPS are becoming more and more expensive
73．The underlined word"They"in paragraph 3means"D"．
A．Receivers
B．GPS devices
C．Satellites
D．Ground stations
74．The passage is mainly aboutB．
A．the history of the GPS
B．the introduction of the GPS
C．the shape of the GPS
D．the three parts of the GPS
75．What can we infer（推斷） from the passage？D
A．All GPS devices have electronic maps．
B．People in many countries will use the GPS for free．
C．The receiver of the GPS contacts at least five of the satellites．
D．The GPS will become more and more common in everyday life．
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