–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
网友回答
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。