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。 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.

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