直流电动机降低电枢电压调速时，转速只能从额定转速( )。 A、升高一倍； B、往下降； C、往上升； D、开始反转；

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直流电动机降低电枢电压调速时，转速只能从额定转速( )。
A、升高一倍；
B、往下降；
C、往上升；
D、开始反转；

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