编写如下代码： Dim Flag As Boolean Private Sub Form MouseDown (Button As Integer, Shift As Integer, X As Single, Y As Single) Flag = True End Sub Private Sub Form MouseMove (Button As Integer, Shift As Integer, X As Single, Y As Single) Flag = False End Sub Private Sub Form_ MouseUp (Button As Integer,Shift As Integer, X As Single, Y As Single) If Flag = False ThenPrint "DDDD" End If End Sub程序运行后，如果想在窗体上输出DDDD，则应执行的操作为______ 。