单击窗体时，下列程序的执行结果是 Private Sub Invert(By Val xstr As String，ystr As String) Dim tempstr AS String Dim I AS Integer I=Len(xstr) Do While I＞=1 tempstr=tempstr + Mid(xstr，I，1) I=I - 1 Loop ystr=tempStr End Sub Private Sub Form_Click( ) Dim s1 As String，s2 As String S1= abcdef Invert S1，S2 Print S2 End Sub
A．abcdef
B．afbecd
C．fedcba
D．defabc