单项选择题

下列程序的执行结果为  Private Sub Commandl_Click()    Dim s1 As String ,s2 As String    s1= "abcd"    Call Transfer(sl,s2)    Print s2  End Sub  Private Sub Transfer (ByVal xstr As String,ystr As String)    Dim tempstr As String    ystr=tempstr  End Sub

A.dcba
B.abdc
C.abcd
D.dabc