直流电机运行在电动机状态时，其( )。 

A、<span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">E</span><span style="font-size: 12.6px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; background-color: rgb(255, 255, 255);">a</span><span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">＞U</span>

B、<span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">E</span><span style="font-size: 12.6px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; background-color: rgb(255, 255, 255);">a</span><span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">＜U　</span>

C、<span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">E</span><span style="font-size: 12.6px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; background-color: rgb(255, 255, 255);">a</span><span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">＝0&nbsp;</span>

D、<span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">E</span><span style="font-size: 12.6px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; background-color: rgb(255, 255, 255);">a</span><span style="font-size: 16.8px; background-color: rgb(255, 255, 255);">＝U</span><span style="font-size: 12.6px; line-height: 0; position: relative; vertical-align: baseline; top: -0.5em; background-color: rgb(255, 255, 255);">&nbsp;</span>