以下代码为什么不能成功？输出为：（有警告）
  X Y Z
1 1 4 1
2 2 5 2
3 3 6 3
Warning messages:
1: In df$Z <- c(df$X, df$Y) :
  number of items to replace is not a multiple of replacement length
2: In df$Z <- c(df$X, df$Y) :
  number of items to replace is not a multiple of replacement length
3: In df$Z <- c(df$X, df$Y) :
  number of items to replace is not a multiple of replacement length