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【B18】
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【B15】
【B19】
听力原文： In the earliest times, men considered lightning to be one of the great mysteries of nature. Some ancient people believed that lightning and thunder were the weapons of the gods. In reality lightning is a flow of electricity from high above the earth.
The American scientist and statesman, Benjamin Franklin, was the first to show the connection between electricity and lightning in 1752. In the same year he also built file first lightning rod. This device protects buildings from damage by lightning.
In the United States alone lightning kills an average of one person every day. The safest place to be in case of an electrical storm is in a closed car. Outside, one should go to low ground and not get under trees. Also, one should stay out of water and away from metal fences. Inside a house, people should avoid opening doors and windows and touching wires or metal things.
What did people once think about lightning？
作业J1，J2，J3，J4的提交时间和运行时间见下表。若采用短作业优先调度算法，则作业调度次序为(18)，平均周转时间为(19)分钟(这里不考虑操作系统的开销)。
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