—Thanks for your help, but can you do me one more favor, please?
— , sir．What's it then?
A. It is nothing
B. That's right
C. At your service
D. Never mind
网友回答
A. It is nothing
B. That's 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。