【单选题】【消耗次数：1】
我国交流电的频率为50Hz，其周期为（ ）秒。
①
0.01
②
0.02
③
0.1
④
0.2
参考答案：
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【单选题】
阳光中的紫外线照射到皮肤上可生成( )。
①
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②
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③
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④
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