单选题 .  Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate 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 timing the interval between a sound and its reflection (the echo), you can estimate the distance to an obstruction.
    During World War Ⅱ the British used a practical application of this principle to detect German planes on their way to bomb London long before the enemy was near the target. They used radio waves instead of sound waves, since radio waves can penetrate fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to the puzzled Germans to be lying in wait at the right time and never to be surprised. It was radio echoes more than anything else that won the Battle of Britain.
    Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them (their range of flight, in other words), the device was called radio directing and ranging, and from the initials the word radar was coined.1.  Sound waves reflected from a hill can be used to estimate the ______.