She is the only one of the girls in our class______ been to the United            States. 

A．who have

B．that had

C．whom has

D．that has

Y，d11接线的变压器，一次A相电压与二次a相电压的相位关系是（）

A、相位相同；

B、一次A相电压超前二次，d相电压π／6；

C、二次a相电压超前一次A相电压π／6；

D、相位相反。