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，单击鼠标左键或右键都能弹出菜单的事件过程是（ ）。

Question

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