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[单选题]
O2和C02进出细胞膜是通过( )
A
单纯扩散
B
易化扩散
C
主动转运
D
入胞作用
E
出胞作用
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试题答案:
A
答案解释:
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视神经记录到的视觉动作电位和听神经上记录到的听觉动作电位( )
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一般血型抗原都是红细胞膜上的(??)
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李商隐《无题》中推己而及对方的一联是( )
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一名专业的小学教师角色包括( )。
3.
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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张某向苏某发出书面要约,并附言:若不承诺应于十天之内答复,不答复视为承诺。苏某收到要约后十天内未答复。应认定()
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简述种间竞争的竞争类型。
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