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，以下( )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键单击。
A．Private Sub Form_Mouse Down(Button As Integer,Shift As Integer,_X As Single,Y As Single)
If Button=2 Then PopupMenu a1,2
End Sub
B．Private Sub Form Mouse Down(Button As Integer,Shift As Integer,_X As Single,Y As Single)
PopupMenu a1,0
End Sub
C．Private Sub Form_Mouse Down(Button As Imeger,Shift As Integer,_X As Single,Y As Single)
PopupMenu a1
End Sub
D．Private Sub Form_Mouse Down(Button As Integer,Shift As Integer,_X As Single,Y As Single)
If (Button=vbLetfButton)Or(Button=vbRightButton)Then PopupMenu a1
End Sub