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A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot 5 how fast nearby planes are moving.

The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the 10 waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300,000 kilometers in one second. A radar set sends out a short burst of radiation waves. Then it receives the echoes produced when the waves bounce off objects. By determining the time it takes for the echoes to return to the radar set, a trained 15 technician can determine the distance between the radar set and other objects. The word “radar,” in fact, gets its name from the term “radio detection and ranging.” “Ranging” is the term for detection of the distance between an object and the radar set. Besides being of critical importance to pilots, radar is essential for air traffic control, 20 tracking ships at sea, and for tracking weather systems and storms.

1. What is the main topic of this passage?

2. In line 2, the word “dense” could be replaced by

3. According to the passage, what can radar detect besides location of objects?

4. The word “shouts” in line 8 is most similar in meaning to which of the following?

5. Which of the following words best describes the tone of this passage?

6. The phrase “a burst” in line 13 is closest in meaning in which of the following?

7. The word “it” in line 13 refers to which of the following?

8. Which of the following could best replace the word “bounce” in line 13?

9. Which type of waves does radar use?

10. The word “tracking” in line 20 is closest in meaning to which of the following?

11. Which of the following would most likely be the topic of the next paragraph?

12. What might be inferred about radar?