---Can I help you?---No thank you. I now.A. have been servedB. have servedC.

问题补充：

------ Can I help you?

-------- No, thank you. I now.

A. have been served

B. have served

C. am being served

D. am serving

答案：

A. have been served

B. have served

(答案→)C. am being served

D. am serving

解析：考察时态语态。根据can I help you?可知，该对话发生在商店里，所以I和serve之间是被动关系，排除BD。因为前面有NO，所以我现在正在有人招待了。