单选题 以下程序的输出结果是(    )。
   Dim s As String,x As String,y As String,z As String
     Dim i As Integer
     s="ABBACDDCBA"’
     For i=6 To 2 Step-2
      x=Mid(s,i,i)
      y=Left(s,i)
      z=Right(s,i)
      z=x&y&z
   Next i
   MsgBox z
   A．AABBAA    B．ABAABB    C．BBABBA D．AABAAB