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[单选题]
下列属于人工辅助能的是( )
A
A.种苗和有机肥料中的化学潜能
B
风
C
潮汐能
D
太阳能
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试题答案:
A
答案解释:
相关题目
在一定范围内,当条件相同时,不管一个种群的密度如何,最后产量差不多都是一样,这属于__________法则。
植物的密度效应指的是 ( )
在个体水平上,生态系统的稳态机制主要通过生理的和__________变化去适应环境的变化。
现代生态学以人为研究中心,更加注重实践研究。( )
在我国暖温带地区,通常土壤养分有效性最好和最利于植物生长的土壤酸碱度为( )
生产力和生物量最大的生态系统类型是( )
简述温度与生物分布的关系。
关于生物群落的特征,下列叙述正确的是 ( )
海洋生态系统包括海岸带、浅海带、上涌带、远海带和 等部分。海带、裙带莱、紫菜在自然状态下生活在海岸带部分。
水稻土属于( )
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11.若一位教师可讲授多门课程,每门课程只有一位教师讲授,则教师实体与课程实体之间的联系类型是
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2.通过搜集和分析研究各种现存的有关文献资料,从中选取信息,以达到某种调查研究目的的方法是
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1.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)1.1 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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行政单位包括( )。
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