现有数据集A<-data.frame(IP=c("001","002","003","002","001"),IC=c("a","b","c","d","e"),IE =c(1,2,3,4,5))B<-as.data.frame(table(A$IP))
根据B，我要从A中找出所有频次为2次的观测，请问我该怎么弄？急需各位大侠帮帮忙