一台他励直流电动机带恒转矩负载运行在额定状态下，现降低电源电压，则电动机转速将          。  

A、<span style="font-size:14px;font-family:宋体">升高</span><span style="font-size:14px;font-family:&#39;Times New Roman&#39;,&#39;serif&#39;">&nbsp;&nbsp;</span>

B、<span style="font-size:14px;font-family:宋体">降低</span>

C、<span style="font-size: 14px;font-family:宋体">不变</span>

D、<span style="font-size:14px;font-family:宋体">不能确定</span>