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在连带责任保证中，主债务消灭或者不成立，则连带责任保证不存在。()
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在个人住房贷款中，抵押物价值的确定以该房产在该次买卖交易中的成交价或评估价的较低者为准。()
房地产开发企业将未解除商品房买卖合同的商品房再行销售给他人时，应办理产权登记。()
To us it seems so natural to put up an umbrella to keep the water off when it rains. But actually the umbrella was not invented as protection against rain. Its first use was as a shade against the sun!
Nobody knows who first invented it, but the umbrella was used in very ancient times. Probably the first to use it were the Chinese, as early as the eleventh century B. C.
We know that the umbrella was used in ancient Egypt and Babylon as a sunshade. And there was a strange thing connected with its use: it became a symbol of honour and authority. In the Far East in ancient times the umbrella was allowed to be used only by royalty or by those in high office.
In Europe, the Greeks were the first to use the umbrella as a sunshade. And the umbrella was in common use in ancient Greece. But it is believed that the first persons in Europe to use the umbrella as protection against the rain were the ancient Romans.
During the Middle Ages, the use of the umbrella practically disappeared. Then it appeared a gain in Italy in the late sixteenth century. And again it was considered a symbol of power and authority. By 1680, the umbrella appeared in France, and later on in England.
By the eighteenth century, the umbrella was used against rain throughout most of Europe. Umbrellas have not changed much in style. during all this time, though they have become much lighter in weight. It wasn' t until the twentieth century that women' s umbrellas began to be made, in a whole variety of colours.
According to this passage, the umbrella was probably first invented ______.
Among the question of design, construction, and operation remaining to be solved before commercial airliners travel faster than sound is the difficult problem of sonic boom (声震) , the explosive sound generated when an object moves through the air at supersonic speed(超音速).
A sound is a pressure disturbance: a sonic boom, like other explosive sounds, is the result of an abrupt change in pressure. An airplane in subsonic flight produces weak changed in pressure, but because these disturbances travel at the speed of sound, they move faster than the airplane and stay in front of it. In effect, these disturbances warn the air to get out of the way, and the an does just jet, however, gets ahead of its own pressure disturbances. Consequently a wave of suddenly com pressed air--a shock wave--builds up and is thrown off like the wave ;hat spreads from the bow of a speedboat.
Can sonic booms be eliminated？ The answer is a categorical negative--they are an innerent(内在的) part of supersonic flight--but the sonic boom probably can be reduced; government and private research is now under way to improve understanding of the sonic boom. If and when the booms can be reduced to tolerable levels, commercial supersonic transports can come rata service.
Before commercial airliners travel faster than sound, problems must 0e solved regarding ______.
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