频率
参考答案：
交流电流1s内电流方向改变的次数称为频率，用字母f表示，单位为Hz（赫兹）。
－How tired we are!
－Yes. ______we have math tests, _______we have physics tests. So many tests.
A．Sometime, at other time
B．Sometimes, at other times
C．Sometimes, at another time
D．Sometime, at another times
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