-– I wonder how much you charge for your services.The first two are free the third cos

问题补充：

-– I wonder how much you charge for your services.

--- The first two are free the third costs $30.

A. while

B. until

C. when

D. before

答案：

A. while

B. until

C. when

D. before

解析：

While这里表示对比关系。这里考查连词引导的状语从句。