(2013·安徽安庆二模)—She looks very happy. She ________ have passed the exam.

—I  guess so. It's not difficult after all.

A．should

B．must

C．could

D．might

对称三相电源三角形连接时，线电流是（）

A、相电流

B、3倍的相电流

C、2倍的相电流

D、√3倍的相电流