假定已经在菜单编辑器中建立了窗体的弹出式菜单，其顶级菜单项的名称为a1，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 Intege, _Shift As Integer, X As Single, Y As Single)     If Button=1 And Button=2 ThenP     opupMenu a1     End If     End Sub
B．Private Sub Form MouseDown(Button As Integeg_Shift As Integer, X As Single, Y As Single)     PopupMenu a1     End Sub
C．Private Sub Form MouseDown(Button As Intege_Shift As Integeg, 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 Button=2 Then     PopupMenu a1      End If     End sub