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

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

admin2011-01-02  33
问题    The idea of a fish being able to produce 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 most known are the electric rays, or torpedoes, of which several kinds live in warm seas. They possess 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 ceils 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 gets through from the upper, positive side of the organ downward 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 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 and are supplied with nerves from the spinal cord. Consequently, the current passes along the fish from head to tail. The electric organs of these fish are really altered muscles and like all muscles are apt to tire, so they are not able to generate electricity for very long. People in some parts of South America who value the electric eel as food, take advantage of this fact by driving horses into the water against which the fish discharge their electricity. The horses are less affected than a man would be, and when the electric eels have exhausted themselves, they can be caught without danger.
The electric catfish of the Nile and of other African fresh waters has a different system again by which current passes over the whole body from the tail to the head. The shock given by this arrangement is not so strong as the other two, but is none the less unpleasant. The electric catfish is a slow,lazy fish, fond of gloomy places and grows to about 1 metre long; it is eaten by the Arabs in some areas.
The power of producing electricity may serve these fish both for defence and attack. If a large enemy attacks, the shock will drive it away ;but it appears that the catfish and the electric eel use their current most often against smaller fish, stunning them so that they can easily be overpowered.  
Why are horses used to catch the electric eels?
选项 A、To frighten the eels.
B、To exhaust the eels.
C、To attract the eels.
D、To stir the water.
答案B
解析 由第四段中的“The horses are less affected than a man would be，and when the electric eels have exhausted themselves，they can be caught without danger．”可知。
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