—I wonder how much you charge（要价） for your services.—The first two are freethe third costs

问题补充：

—I wonder how much you charge（要价） for your services.

—The first two are freethe third costs￥30.

A. while

B. until

C. when

D. before

答案：

(答案→)A. while

B. until

C. when

D. before

解析：这是一组对话。上文，我想知道你的服务收费多少？下文，前两次免费，然而第三次收费30元。While然而 until到……为止 when 当……时 before 在……之前，根据题意，故选A。