在窗体上画—个命令按钮(名称为Command1)，并编写如下代码：Function Fun1 (ByVala As Integer, b As Integer)As Integer Dim t As Integer t=a-b b=t+ a Fun 1=t+ bEnd FunctionPrivate Sub Command1_ Click() Dim x As Integer x=10 Print Fun1(Fun1 (x,(Fun1(x，x-1))),x-1)End Sub程序运行后，单击命令按钮，输出结果是 ______。

A．10

B．0

C．11

D．21