听力原文：Under proper conditions, sound waves will be reflected from a hillside or other s

听力原文： Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the speed of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus by measuring the time between a sound and its reflection-the echo, you can calculate the distance to an obstruction.

During World War II the British made a practical use of this rule to discover planes on their way to bomb London long before the enemy was near the object. They used radio waves instead of sound waves, since radio waves can pass through fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to puzzle Germans that they were lying in wait at the right time and were never surprised. It was radio echoes more than anything else that helped them win the war.

Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them, the device was called radio directing and ranging, and from the first letters the word radar was formed.

Sound waves reflected from a hill can be used to calculate the______.

A．Speed of sound

B．time between a sound and ifs echo

C．height of the hill

D．distance of the hill