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

A.fedcba
B.abcdef
C.afbecd
D.defabc