单项选择题

下列程序的执行结果为
Private Sub Command1_C1ick( )
Dim sl As String，s2 AS String
s1="abcdef"
Call lnvert(s1，s2)
Print s2
End Sub
Private Sub lnvert(ByVal xstr As String，ystr As String)
Dim tempstr As Stdng
i=Len(xstr)
Do While i＞=1
tempstr=tempstr+Mid(xstr，i，1)
i=i-1
Loop
ystr=tempstr
End Sub