问题：
[单选题]一台三相异步电动机拖动额定负载转矩起动时，若电源电压下降了15％，这时电动机的电磁转矩（ ）。
A、Tem=TN
B、Tem=0.7225TN
C、Tem=0.85TN
D、Tem=0.75TN
参考答案：
B
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