脉宽调制（PWM）的基本原理是通过开关管把直流电斩成方波（脉冲波），通过调解波的占空比（脉冲宽度与脉冲周期之比）来改变电压。
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欲结束快充充电时无需进行充电结束刷卡或手机APP结束充电操作,直接拔出快充枪即可。
A. 对
B. 错
I _____ an old friend of mine while I _____in the street yesterday afternoon.
A. met ... was walking
B. was meeting ... walked
C. was meeting ... was walking
D. met ... walked
One day when I ____the post office, I _____my uncle.
A. pass ... saw
B. was passing ... saw
C. passed ... was seeing
D. am passing ... am seeing
The student _____to move the bag of rice _____he heard the sound of a motorbike.
A. was running ... when
B. was running ... while
C. runs ... when
D. ran ... while
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