题干

如表列出了四种导线材料的单价、密度和用这些材料制成长1m、横断面积1mm2的导线在20℃时的电阻值:(说明:“$•t 1”表示“美元/吨”)

导线材料

单价/(•t1

密度ρ/(kg•m3

电阻r/Ω

10000

10.5×10 3

0.016

1842

8.9×10 3

0.017

1461

2.7×10 3

0.027

1050

7.9×10 3

0.096

要架设一条远距离输电线路,经综合考虑,应选用的导线材料是(  )

A:银

B:铜

C:铝

D:铁

上一题 下一题 0.0难度 选择题 更新时间:2018-06-20 05:09:28

答案(点此获取答案解析)

B

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      Students these days often have a lot of worries. Sometimes they have problems with their schoolwork, and sometimes with their friends.      Some people think the worst thing is to do nothing. Laura Mills, a teenager from Washington, agrees. “Problems and worries are normal in life,” says Laura. “But I think talking to someone helps a lot. If we don't talk to someone, we'll certainly feel worse.”
    Laura once lost her purse, and worried for days. She was afraid to tell her parents shout it. She even walked three miles to school each day because she didn't have any money. She just kept thinking. “If I tell my parents, they'll be angry!” But in the end, she talked to her parents and they were really understanding. Her dad said he sometimes made careless mistakes himself. They got her a new purse and asked her to be more careful. “I will always remember to share my problems in the future!” Laura says.
       Robert Hunt advises students about common problems. He feels the same way as Laura. “It is best not to avoid our problems. We should always try to solve (解决) them.” He thinks that you can first find someone to talk to. This person doesn't need to be an expert (专家) like himself. Students often forget that their parents have more experience than them, and are always there to help them.
    In English, we say that sharing a problem is like cutting it in half. So you're halfway to solving a problem if you talk to someone about it!