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

问题补充：

–I wonder how much you charge for your service.

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

A. before

B. as

C. when

D. while

答案：

A. before

B. as

C. when

(答案→)D. while

解析：连词辨析。Before在……之前；as因为，尽管；when当……时候；while

然而，表前后对比。句子是说前两个免费而第三个30美元。表示前两个和第三个对比，所以选D。