若数列{an}的前n项和Sn=n2+2n+5，则a5+a6+a7=（ ）。
39
—Is the man in blue our headmaster?
—It _____ be him. He has just left for Beijing. [ ]
A. must
B. may
C. can't
D. couldn't
信息资源管理标准化工作要遵循哪些指导原则( )
A．效益
B．系统
C．动态
D．协商
E．优化
