—I wonder how much you charge for your services.—The first two are free the third costs $

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：

(答案→)A. while

B. until

C. when

D. before

解析：while是并列连词，表示对比，意为“而”。