C

　　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 move about in dim candlelight， cars hesitate in the streets because there are no traffic lights to guide them， and food goes bad in silent refrigerators.

　　Yet people began to understand how electricity works only a little more than two centuries ago. Nature has in fact been experimenting in this field for millions of years. Scientists are discovering more and more that the living world may hold many interesting secrets of electricity that could benefit humanity.

　　All living cells send out tiny pulses（脉冲） of electricity. As the heart beats， it sends out pulses that can be measured and recorded on the surface of the body. When the pulses are recorded， 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 an electroencephalogram. The electric currents generated by most living cells are very small　—　often so small that sensitive instruments are needed to record them. But in some animals， certain muscle cells have become so specialized as electrical generators（发电机） that they do not work as muscle cells at all. When large numbers of these cells are linked together， the effect can be astonishing.

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