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Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
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 I1 the British used a practical application of this principle to detect German planes on their way to bomb London 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 (袭击). 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 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.
Sound waves reflected from a hill can be used to estimate______.
Throughout history, man has had to accept the fact that all living things must die, but people now live longer than they used to. Yet, all【C1】______things still show the effect of aging, which will eventually【C2】______death. The body and the【C3】______, they do not function as well as they【C4】______in childhood and adolescence(青春期). The body provides less【C5】______against disease and is more prone(易于……的)【C6】______accident.
A number of related causes may【C7】______to aging. Some cells of the body have a【C8】______long life, but they are not【C9】______when they die. As a person ages,【C10】______of brain cells and muscle cells decreases. Other body cells did and are replaced by new cells. In an aging person the【C11】______cells may not be as viable(能生存的)or as capable【C12】______growth as those of a young person.
Another factor in aging may be changes within the cells【C13】______. Some of the protein chemicals in cells【C14】______known to change【C15】______age and become less elastic. This is why the skin of old people wrinkles and hangs loose. This is also the reason old people shrink in【C16】______. There may be other more important chemical changes in the cells. Some complex cell chemicals.【C17】______DNA and RNA, store and【C18】______information that the cells need. Aging may【C19】______this process and change the information carrying molecules【C20】______they do not transmit the information as well.
【C1】
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