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.

(11)

A．through

B．for

C．on

D．against.