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

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：这里起转折作用,while表示”但是”,可接句子.”前两个免费,但第三个要收费30元”