题目

While ____ in tea with that dealer, Mr. Smith had himself ____． A．trading; ripped off                              B．buying; ripping off C．selling; ripped off                               D．purchasing; rip off 答案：ASn是数列{an}的前n项和，且a1=1，an+1=an+2，则S5=（　　）A．40B．35C．30D．25