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  3. 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 us

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 us

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问题     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 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 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 radio 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 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, tracking ships at sea, and for tracking weather systems and storms.  
According to the passage, what can a radar detect besides location of objects?
选项 A、Size.
B、Weight.
C、Speed.
D、Shape
答案C
解析 细节题型见本文第三行：Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. (由于人眼并不擅长判定物体接近的速度，雷达就被用来给飞行员显示附近的飞机速度有多快)；因此可知雷达除了可探测物体的方位，还能探测物体的速度，选项C为答案。
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