单项选择题

编写如下事件过程:   Private Sub Form_MouseDown(Button AS Integer,Shift AS Integer,_                 X As Single,Y As Single)     If Shift=6 And Button=2 Then       Print,"PC"     End If   End Sub   程序运行后,为了在窗体上输出“PC”,应执行的操作为( )。

A.主索引
B.候选索引
C.惟一索引
D.普通索引