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。