Choose the option that best completes each blank in the passage. Write A, B, or C in the blank.

GPS and Survival

The USA (21) _________ the Global Positioning System (GPS) in the 1970s. At first, it was a military project, but now anyone can use the system to find out exactly their locations, and (22) _________ their movements.

For GPS to work, your receiver (or smartphone) must be in contact with four or more satellites in space. Then it uses the data to calculate the exact time and (23) _________ on Earth.

However, it is not enough for survival situations (escaping the jungles or deserts) because you must know your direction - which way you are moving and your speed - how (24) _________ you are moving. GPS provides this information, by tracking your position over time.

For modern explorers, a smartphone and GPS are as important as a compass, torch and a map. With the right applications, your (25) _________ will help you survive.