执行下面的程序，消息框里显示的结果是______。
Private Sub Form_Click()
Dim Str As String,S As String,k As Integer
Str="abc"
S=Str
For k=Len(Str) To 1 Step -1
S=S & (Mid(Str,k,1)
Next k
MsgBox S
End Sub

十二烷基苯磺酸钠是表面活性剂。