执行下列程序后，鼠标单击窗体，输出结果为( )。 Private Sub Form_Click() Print "Click"; End Sub Private Sub Form_MouseDown(Button As Integer, Shift A: As Integer,X As Single,Y As Single) B: Print "Down" C: End Sub D: Private Sub Form_MouseUp(Button As Integer, Shift_As Integer,X As Single,Y As Single) E: Print "Up" F: End SubA) DownUpClick G: B) ClickDownUp H: C) DownClickUp I: D) UpDownClick