Private Sub Option1_Click(Index As Integer)
    Dim n As Integer
    n = Val(Text1.Text)
'    Select Case ?
        Case 0
'            Text2.Text = f1(?)
        Case 1
'            Text2.Text = f1(?)
    End Select
End Sub
Public Function f1(n As Integer) As Integer
    Dim x As Long
    x = 1
'    For i = 1 To ?
      x = x * i
    Next
'    ? = x
End Function
Private Sub Text2_Change()
End Sub