单选题：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al，其“可见”属性为False。程序运行后，单击鼠标左

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al，其“可见”属性为False。程序运行后，单击鼠标左键或右键都能弹出菜单的事件过程是（　　）。 A．Private Sub Form_MouseDown(Button As Integer，_Shift As Integer，X As Single，
Y As Single)
If Button=1 And Blltton=2Then
PopupMenu a1
End If
End Sub
B．Private Stlb Form_MouseDown(Button As Integer，_Shift As Integer，X As Single，
Y As Single)
PopupMenu a1
End Sub
C．Private Sub Form—MouseDown(Button As Integer，_Shift As Integer，X As Single，
Y As Single)
If Button=1 Then
PopupMenu a1
End If
End Sub
D．Private Sub Form MouseDown(Button As Integer，_Shift As Integer，X As Single，
Y As Single)
If Buttcn=2 Then
PopupMenu a1
End If
End Sub