运行代码：manager<-c(1,2,3,4,5)date<-c("10/24/08","10/28/08","10/1/08","10/12/08","5/1/09")
country<-c("US","US","UK","UK","UK")
gender<-c("M","F","F","M","F")
age<-c(32,45,25,39,99)
q1<-c(5,3,3,3,2)
q2<-c(4,5,5,3,2)
q3<-c(5,2,5,4,1)
q4<-c(5,5,5,NA,2)
q5<-c(5,5,2,NA,1)
leadership<-data.frame(manager,date,country,gender,age,q1,q2,q3,q4,q5,stringAsFactors=FALSE)
attach(leadership)     #这时会报以下错误

The following objects are masked _by_ .GlobalEnv:

    age, country, date, gender, manager, q1, q2, q3, q4, q5


我想请问这是什么原因？？？？？