1. How did the early explorers know where they were? They found out where they were by using stars. 2. What’s the disadvantage of using the stars as guiding marks? Its disadvantage is that it didn’t work well when the weather was bad and the stars could not be seen. 3. What is a GPS? Is it useful to use? It is like a man-made star. We can use it at any time, in any place and in any weather to find out our position. It is very useful. 4. What do you think the GPS will be used for in the future? …

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