下列短文中有十个空白，每个空白有四个选项。根据上下文要求选出最佳答案，并在答题纸上将相应的字母涂黑。

　　The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting 11 the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of 12 _ waves. Radio waves travel 13 the speed of light, about 300,000 kilometers in a second. A radar set _ 14 _ a short burst of radio waves. Then it receives the echoes _ 15 when the waves bounce (反射) off objects. By determining the time it 16 for the echoes to return to the radar set, a trained technician 17 determine the distance between the radar set and other objects. The word '6radar", 18 , gets its name from the term "radio detection and ranging." "Ranging" is the term for detection of the 19 between an object and the radar set.

　　Besides being of critical importance to pilot, radar is 20 for air traffic control, for tracking ships at sea, and for tracking weather systems and storms.

　　11. A. through B. for C. on D. agamsr

　　12.A.sound B. light C. heat D. tide

　　13.A.in B. at C. over D. on

　　14. A. sends out B. works out C. figures out D. puts out

　　15. A. producing B. produced C. produce D. produces

　　16.A. spends B. takes C. kills D. makes

　　17.A.can B. must C. ought to D. has to

　　18.A.in case B. in turn C. in short D. in fact

　　19.A.length B. width C. distance D. space

　　20. A. eventual B. internal C. external D. essential

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