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 Then
PopupMenu 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