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[单选题]
单种种群在无限制的环境中呈指数增长时,其增长率为( )
A
变量
B
恒值
C
负值
D
零值
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参考答案
试题答案:
B
答案解释:
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土壤细菌和豆科植物的根系所形成的共生体称为( )
种群增长包括正增长、负增长和__________。
高纬度地区恒温动物的身体住往比生活在低纬的同类个体大,因为个体大的动物,其单位体重散热量相对较少,这就是 。
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Directions:For each blank in the followring passage, there are four choices maked A, B, Cand D. Choose the one that is most suitable and mark your answer by blackening the corres rponding letter on the Answer Sheet.Where do cars get their energy from? For most cars, the answer is petro1. 21.some cars use electricity. These cars have 22_motors that get their power from large batteries.In 23,there are even cars that have 24_an electric motor and a petrol motor. These types of cars are 25hybrid(混合) cars.Most people tend to think of electric cars as a new 26but they have been around for a 1ong time. In the 27_19th and early 20th centuries electric cars were common because the technology for petrol engines was not very advanced. But 28_the petrol engine became easier to make and more powerful, this type of engines became the most 29. Interest in electric cars was high in the 1970s and 1980s because 30_became very expensive.Recently; electric cars have again become wel1- liked because people want cars that pollute 31 Electric cars are better than petrol cars 32_several ways. The biggest benefit is reduced pollution.In areas 35 there is a high percentage of electric cars, pollution is not that serious. The second benefit of electric cars is a 34_in the dependence on foreign oi1.Several countries don’ t want to 3!on oil from other countries. Since electric cars can run on electricity from coal or nuclear power stations, there is less need to import oil.
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3. Friction is a force which opposes motion. Much of the energy supplied to a complicated machine with many moving parts is wasted in overcoming this opposition. The energy is converted into useless heat. The amount thus lost can be considerably reduced and the life of the machine prolonged by careful lubrication. This reduction improves the efficiency of the machine. Another way in which friction is reduced is to substitute roller or ball bearings for the earlier type of bearing, in which the surfaces in contact move by sliding. Bearings of these later types are used very extensively in many kinds of modern machinery.It must not be assumed, however, that friction is always a disadvantage. In many instances it is highly important. Belts are often used in conveying power from one machine to another. In the absence of friction, this could not be done. The driving wheel would not run the belt, neither would the belt run the wheel to driver. Without friction, then, belted machinery could not be used. Again in the case of all motor vehicles, friction is essential in the operation of the brake. Were there no friction, there could be no brake. But even this illustration does not give an adequate picture of the importance of friction. The vehicle could not even be started if it were not for the friction exerted between the tires and the road. Nails hold in wood, and screws hold in either wood or metal by reason of friction, without which they would be useless. Walking is difficult enough on an icy pavement, but it would be wholly impossible if there were no friction at all between the shoe and the pavement.(5分)3.1 Friction arises when____.(1分)A. an opposing force of motion exists B. the energy is wastedC. everything is motionlessD. a complicated machine is supplied with energy
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