单项选择题


Throughout history man has had to accept the fact that all living things must die, but people now live longer than they (51) Yet, all living things still show the (52) of aging, which will eventually (53) death.
Aging is not a disease, but as a person passes maturity, the cells of the body and the (54) they form do not function (55) they did in childhood and adolescence. The body provides less (56) against disease mad is more (57) accident.
A number of related causes may (58) aging. Some cells of the body have a fairly long life, but they are not (59) when they, die. As a person ages, (60) of brain cells and muscle cells decreases. (61) body cells die and are replaced by new cells. In an aging person the (62) cells may not be as viable or as (63) growth as those of a young person.
Another (64) in aging may be changes within the cells (65) Some of the protein chemicals in cells are known (66) with age and become less elastic. This is why the skin of old people (67) and hangs loose. This is also the reason old people (68) in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and (69) information that the cells need. Aging may affect this (70) and change the information-carrying molecules so that they do not transmit the information as well.

A．a number
B．an amount
C．tile number
D．most
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单项选择题
A．replacedB．rebornC．recoveredD．recuperated
A．replaced
B．reborn
C．recovered
D．recuperated
问答题
One of nature’s most destructive forces is the tornado, a violent windstorm that takes the shape of a rotating column of air. Tornadoes almost always occur in conjunction with severe thunderstorms that produce high winds, heavy rainfall and damaging hail. Though their cause is unknown, tornadoes are believed to be the result of the convergence of strong upward wind currents inside a storm with upper level winds above the storm; the greater the air contrast, the more violent the storm will be. The United States has more tornadoes, approximately 750 tornadoes reported each year, than any other country in the world because this is where arctic and tropical air masses most frequently converge.This unpredictability makes accurate tornado forecasts difficult. Though it is possible to determine when a tornado is apt to occur, actual tornado warnings are issued only when a tornado has been sighted or reported on radar. Radar can be used to guess the storm’s’ likely path, its speed and the intensity of the storm. But conventional radar has limitations. An advanced form of radar, known as Doppler, has the ability to detect the first steps in the formation of a tornado. Unlike conventional radar, Doppler tracks a thunderstorm’s rotating wind system which usually precedes the development of a tornado. As a result, Doppler has provided forecasters with the ability to issue tornado warnings as much as 20 minutes prior to a storm’s touchdown, compared to a warning of less than 2 minutes by visual sighting. Though tornadoes remain one of nature’s most violent forces, the use of radar and advanced warning systems has substantially narrowed their paths of destruction.
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