设已经在菜单编辑器中设计了窗体的快捷菜单，某顶级菜单为a1，且取消其“可见”属性。运行时，哪个事件过程可以使快捷菜单的菜单项响应鼠标左键单击和右健单击的事件过程是
A．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single) If Button=2 Then PopupMenu al,2 End Sub
B．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single PopupMenu al,0 End Sub
C．Private Sub Form_MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single PopupMenu al End Sub
D．Private Sub Form MouseDown(Button As Integer，Shift As Integer,_X As Single，Y As Single If(Button=vbLetfButton)Or(Button=vbRightButton)Then PopupMenu al End Sub