—Thanks for your help but can you do me one more favor please? — sirWhat's it then?A.

问题补充：

—Thanks for your help, but can you do me one more favor, please?

—, sir．Whats it then?

A. It is nothing

B. Thats right

C. At your service

D. Never mind

答案：

A. It is nothing

B. Thats right

(答案→)C. At your service

D. Never mind

解析：考查交际用语。A没有什么大不了的；B那是对的；C乐意为您服务；D没有关系；句意：—谢谢你的帮助，可你能再帮我一个忙吗？—乐意为你效劳。根据句意说明C正确。