正弦交流电的实际方向总是随时间变化而变化的，一般先选定一个参考方向，正半周时实际方向和参考方向一致，负半周时实际方向和参考方向相反。
查看答案
交流电的有效值是指交流电能达到的最大值，用字母Im和Um表示。
A. 对
B. 错
常说的工频电的周期是0.02秒。
A. 对
B. 错
Questions 1 and 2 are based on the news report you have just heard.News 1[音频]Question 1
A. The majority of drivers prefer to drive and park themselves.
B. Human drivers become easily distracted or tired while driving.
C. Most drivers feel uncertain about the safety of self-driving cars.
D. Most drivers have test driven cars with automatic braking features.
Question 2
A. Their drivers would feel safe after getting used to the automatic devices.
B. They would be unpopular with drivers who only trust their own skills.
C. Their increased comfort levels have boosted their sales.
D. They are not actually as safe as automakers advertise.
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