单项选择题

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