在窗体上画一个名称为CD1的通用对话框，并有如下程序：
Private Sub Form_Load()
CD1. DefaultExt="doc"
CD1. FileName="c:\file1.txt"
CD1.Filter="应用程序(*.exe)|*.exe"
End Sub
程序运行时，如果显示了“打开”对话框，在“文件类型”下拉列表中的默认文件类型是( )。

A．Private Sub Form_MouseDown(Button AS Intege, _Shift As Integer, X As Single, Y As Single)     If Button=1 And Button=2 ThenP     opupMenu 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

A．MouseUp事件是鼠标向上移动时触发的事件
B．MouseUp事件过程中的x，y参数用于修改鼠标位置
C．在MouseUp事件过程中可以判断用户是否使用了组合键
D．在MouseUp事件过程中不能判断鼠标的位置