51. In order to thank you for your help, I want to treat you to lunch ________.
A. in return B. by accident C. as well D. on duty
【答案】A
【解析】
【详解】考查介词短语词义辨析。句意：为了感谢你的帮助，我想请你吃午饭作为回报。A. in return作为回报；B. by accident意外地；C. as well也；D. on duty值班，值日。根据前文“In order to thank you for your help”可知，因为表示对别人的感谢，所以此处需用in return，表示请吃午饭作为回报。故选A。