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作者： 　 發(fā)布時(shí)間：2008-08-11 10:59:39　 來(lái)源：育路外語(yǔ)考試頻道
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　Large animals that inhabit the desert have evolved a number of adaptations for reducing the effects of extreme heat.One adaptation is to be light in color,and to reflect rather than absorb the Sun's rays.Desert mammals also depart from the normal mammalian practice of maintaining a constant body temperature.Instead of trying to keep down the body temperature deep inside the body,which would involve the expenditure of water and energy,desert mammals allow their temperatures to rise to what would normally be fever height,and temperatures as high as 46degrees Celsius have been measured in Grant's gazelles.The overheated body then cools down during the cold desert night,and indeed the temperature may fall unusually low by dawn,as low as 34degrees Celsius in the camel.This is an advantage since the heat of the first few hours of daylight is absorbed in warming up the body,and an excessive buildup of heat does not begin until well into the day.
　　Another strategy of large desert animals is to tolerate the loss of body water to a point that would be fatal for non-adapted animals.The camel can lose up to 30percent of its body weight as water without harm to itself,whereas human beings die after losing only 12to 13percent of their body weight.An equally important adaptation is the ability to replenish this water loss at one drink.Desert animals can drink prodigious volumes in a short time,and camels have been known to imbibe over 100liters in a few minutes.A very dehydrated person,on the other hand,cannot drink enough water to rehydrate at one session,because the human stomach is not sufficiently big and because a too rapid dilution of the body fluids causes death from water intoxication.The tolerance of water loss is of obvious advantage in the desert,as animals do not have to remain near a water hole but can obtain food from grazing sparse and far-flung pastures.Desert-adapted mammals have the further ability to feed normally when extremely dehydrated,it is a common experience in people that appetite is lost even under conditions of moderate thirst.
　　29.What is the main topic of the passage?
　　(A)Weather variations in the desert
　　(B)Adaptations of desert animals
　　(C)Diseased of desert animals
　　(D)Human use of desert animals.
　　30.According to the passage,why is light coloring an advantage to large desert animals?
　　(A)It helps them hide from predators.
　　(B)It does not absorb sunlight as much as dark colors.
　　(C)It helps them see their young at night
　　(D)It keeps them cool at night.
　　31.The word "maintaining"in line 4is closest in meaning to
　　(A)measuring
　　(B)inheriting
　　(C)preserving
　　(D)delaying
　　32.The author uses of Grant's gazelle as an example of
　　(A)an animal with a low average temperature
　　(B)an animal that is not as well adapted as the camel
　　(C)a desert animal that can withstand high body temperatures
　　(D)a desert animal with a constant body temperature
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