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 Integer, _Shift As Integer, X As Single,Y As Single)If Button=1 And Button=2 ThenPopupMenu a1End IfEnd Sub
B、 Private Sub Form_MouseDown(Button As Integer, _Shift As Integer, X As Single,Y AS Single)PopupMenu a1End Sub
C、 Private Sub Form_MouseDown(Button As Integer, _Shift As Integer, X As Single,Y As Single)If Button=1 ThenPopupMenu a1End IfEnd Sub
D、 Private Sub Form MouseDown(Button As Integer, _Shift As Integer, X As Single,Y As Single)If Button=2 ThenPopupMenu a1End IfEnd Sub

【正确答案】 B

【答案解析】[解析] 表示窗体的弹出式菜单使用PopupMenu，而单击鼠标的左键或者右键都能弹出菜单，使用的是MoseDown，并且使用Button控制按下的鼠标键Button=1表示的是单击左键，Button=2表示的是单击右键，所以当Button=1或者Button=2时才能实现此功能，所以答案选择B。