金属是最常用的导体，钢铁是导体，又多又便宜，但是最常用铜和铝做导线，大规模的室外架空导线常用铝线，往往采用几股铝线绞在一起，而且中心还加一根钢丝．如图所示，你知道为什么要这样做吗？
按照导电性能来选择，用银做导体最好，但太昂贵．用钢铁做导体虽然又多又便宜，但导电性能较差，又容易生锈．所以导体的材料多选用铜和铝，安装室内导线、用量较小时，常用铜．室外导线，尤其是大规模的架空电线，更多选择铝，因为铝不仅导电性能较好，而且密度小、轻，至于中心加钢丝，则是为了提高电线的抗拉能力．
阅读下面的短文，根据短文内容填写表格，将信息补充完整。每空限填1个单词。 Katie Spotz is a 22-year-old girl from America. She wants to cross the Atlantic Ocean by boat. If she succeeds, she’ll be the youngest and the first American to finish the 2500-mile trip. Spotz began sailing when she was at university, but she has never tried to sail such a long way before. It will be a great challenge for her. In recent years, she has swum all the way down the 325-mile Allegheny River and traveled by bike form Seattle to Washington. Spotz knows that she may see some dangerous things during the trip. It will be very dangerous if she meets very bad weather in the middle of the sea. But she believes that she can deal with all the difficulties. “I never thought I could run 26 miles,” she said. “But once I finished it, I realized that maybe I could do something even more difficult.” Spotz wants to collect money by going on this trip. She wants to collect money for an organization called “Blue Planet Run”. The organization offers free, safe drinking water to people who need it. It will take her about three months to finish this trip. Good luck to this brave girl and best wishes to her!
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