假定已经在菜单编辑器中建立了窗体的弹出式菜单，其顶级菜单的名称为a1，其“可见”属性为False，则程序运行后，可以同时响应鼠标左键单击和右键单击的事件过程是 ______ 。

A．Private Sub Form_ MouseDown (Button As Integer, _ Shift As Integer, X As Single, Y As Singl If Button = 1 And Button = 2 Then PopupMenu al End If End Sub
B．Private Sub Form_ MouseDown (Button As Integer, _ Shift As Integer, X As Single, Y As Singl PopupMenu a1 End Sub
C．Private Sub Form_ MouseDown (Button As Integer, _ Shift As Integer, X As Single, Y As Singl If Button = 1 Then PopupMenu a 1 End If End Sub
D．Private Sub Form_ MouseDown(Button As Integer, _ Shift As Integer, X As Single, Y As Singl If Button = 2 Then PopupMenu a 1 End If End Sub