单击窗体时，下列程序的执行结果是( )。
Private Sub Invert(By Val xstr As String，ystr As String)
Dim tempstr AS String
Dim I AS Integer
I=Len(xstr)
Do While I＞=1
tempstr=tempstr + Mid(xstr，I，1)
I=I - 1
Loop
yStr=tempStr
End Sub
Private Sub Form_Click( )
Dim s1 As String，s2 As String
S1="abcdef"
Invert S1，S2
Print S2
End Sub