问题补充：
The singer sang one more song at the of enthusiastic fans.
A. command
B. recognition
C. request
D. service
答案：
A. command
B. recognition
(答案→)C. request
D. service
解析：at the request of sb 在某人的要求下 A. command“命令”；B. recognition“认知”；D. service“服务”，所以答案为C
