2. 自考
  3. The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting (11)______ the

The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting (11)______ the

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问题      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，about300,000 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“radar”，  (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.   
选项 A、can
B、must
C、ought to
D、has to
答案A
解析 仅仅表示一种“能”的意愿、能力。故选A。
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