Under proper conditions, sound waves will be reflected from a hillside or other such obstruction(障碍). 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, you can estimate the distance to an obstruction.

　　During World War II the British used a practical application(应用) of this law 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 make a way through fog and clouds. The outnumbered Royal Air Force always seemed to the puzzled Germans to be surprised.

　　It was radio echoes(回声) more that 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 line of flight, in other words), the device was called radio direction and ranging, and from the initials the word radar was invented.

　　1. Sound waves reflected from a hill can be used to calculate the _____.

　　A. height of the hill B. speed of sound

　　C. distance to the hill D. intensity of sound

　　2. Radar enabled the English to ______.

　　A. prevent German planes being on their may to London

　　B. direct the outnumbered RAF planes effectively

　　C. confuse German bomber pilots

　　D. number the Royal Air Force

　　3. The British used radio waves because they ______.

　　A. were more exact than sound waves B. could not be detected

　　C. were more useful that sound waves D. were easier to use than sound waves

　　4. The author of this article probably intended to explain ______.

　　A. exactly how radar works

　　B. why the British used radio waves in their device

　　C. How radar (word and device) Came to be

　　D. How radar helped the British win the Battle of Britain

　　答案：CBCC