Reading Comprehension 
从下列每篇短文的问题后所给的四个选择项中选出一个最佳答案。

     The modern age is an age of electricity. People are so used to electric lights, radios, televisions and telephones that it is hard to imagine what life would be like without them. When there is a power failure, people grope about in the dark, cars hesitate in the streets because there are no traffic lights to guide them and food spoils in silent refrigerators.

     Yet, people began to understand how electricity works only a little more than two centuries ago. Nature has apparently been experimenting in this field for millions of years. Scientists are discovering that the living world may hold many interesting secrets of electricity that could benefit humanity.
     All living cells send out tiny pulses of electricity. As the heart beats, it sends out electric pulses. They form an electrocardiogram（心电图）, which a doctor can study to determine how well the heart is working. The brain, too, sends out brain waves of electricity, which can be recorded in electroencephalogram（脑电图）. The electric currents produced by most living cells are extremely small—often so small that sensitive instruments are needed to record them. But in some animals, certain muscle cells have become so specialized in producing electricity that they do not work as muscle cells any more. When large numbers of these cells are linked together, the effects can be astonishing.
     The electric eel—a long thin fish with slippery skin—can produce amazingly strong electricity. It can send an electric current as high as eight hundred volts through the water in which it lives. As many as four-fifths of all the cells in the electric eel's body are capable of producing electricity, and the strength of the shock depends on the length of its body.