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  3. The idea of a fish being able to generate electricity strong enough to light lamp bulbs—or even to run a small electric motor—is

The idea of a fish being able to generate electricity strong enough to light lamp bulbs—or even to run a small electric motor—is

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问题     The idea of a fish being able to generate electricity strong enough to light lamp bulbs—or even to run a small electric motor—is almost unbelievable, but several kinds of fish are able to do this. Even more strangely, this curious power has been acquired in different ways by fish belonging to very different families.
    Perhaps the best known are the electric rays, or torpedoes(电鱼), of which several kinds live in warm seas. They posses on each side of the head, behind the eyes, a large organ consisting of a number of hexagonal shaped cells rather like a honeycomb. The cells are filled with a jelly-like substance, and contain a series of flat electric plates. One side, the negative side, of each plate, is supplied with very fine nerves, connected with a main nerve coming from a special part of the brain. Current passes from the upper, positive side of the organ downwards to the negative, lower side. Generally it is necessary to touch the fish in two places, completing the circuit, in order to receive a shock.
    The strength of this shock depends on the size of the fish, but newly born ones only about 5 centimeters across can be made to light the bulb of a pocket flashlight for a few moments, while a fully grown torpedo gives a shock capable of knocking a man down, and, if suitable wires are connected, will operate a small electric motor for several minutes.
    Another famous example is the electric eel. This fish gives an even more powerful shock. The system is different from that of the torpedo in that the electric plates run longitudinally(纵向)and are supplied with nerves from the spinal(脊骨)cord. Consequently, the current passes along the fish from head to tail. The electric or gans of these fish are really altered muscles and like all muscles are apt(likely)to tire, so they are not able to produce electricity for very long.
    The power of producing electricity may serve these fish both for defence and attack.
The main idea of the third paragraph is that______.
选项 A、the mass of the fish decides the intensity of electric power it generates
B、the strength of shock given by a young electric ray can only light the bulb of a pocket flashlight
C、to make full use of the energy produced by electric fish, suitable wires should be available
D、a mature torpedo is capable of producing enough electricity to knock down a man
答案A
解析 本段第一句说明的主旨，即电击的强度取决于鱼的尺寸。可知A项最合适。
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