在窗体上画一个名称为 Command1的命令按钮，然后编写如下事件过程： Private Sub Command1_Click() Dim a(4)As Integer, x As Integer, sum As Integer n = 4: sum = 0 a (1) = 1 For k = 0 To n - 1 For i = 1 To k + 1 x = k + 2 - i a(x)= a(x)+ a(x - 1) If x ＜ n - k Then Exit For sum = sum + a(x) Next i Next k Print sum End Sub 程序运行后，单击命令按钮，则窗体上显示的内容是

A．5

B．6

C．8

D．11