有如下程序代码: Private Sub Form_MouseDown(Button As Integr ,Shift As Integer , X As Single,Y As Single) FillColor=QBColor(Int(Rnd*15)) FillStyle=Int(Rnd*8) Circle(x,y),250 End Sub 该程序的功能是: A. 双击鼠标时在窗体中构造一 个圆 B. 鼠标拖曳时在窗体中构造一个圆 C. 加载时在窗体中构造一个圆 D. 单击鼠标时在窗体中构造一 个圆