下列程序的执行结果为（ ）。
Private Sub Command1_Click()
Dim s1 As String，s2 As String
S1；="abcdef"
Call Invert(s1，s2)
Print s2
End Sub
Private Sub Invert (ByVal xstr As String，ystr As String)
Dim tempstr As String
i=Len(xstr)
Do While i>=1
tempstr=tempstr+Mid(xstr，i，1)
i=i-1
Loop
ystr=tempstr
End Sub