—Let's have a game of tennis; the loser has to treat the other to a hot dog.

—________.[ ]

A. I'm afraid so  

B. It's a deal

C. You've got a point  

D. My treat

变压器采用Y/△接线方式可以起到（）的作用。

A、消除谐波

B、保证频率质量

C、限制电压波动

D、保证变压器高压侧电压恒定