The school rules state that no child ___ be allowed out of the school during the day , unless accompanied by an adult .

A．may

B．dare

C．shall

D．must

发电机异步运行后的危害（）。

A、转子发生过热；

B、系统电压稳定水平降低；

C、定子绝缘损坏；

D、系统电压升高。