在窗体上画一个命令按钮，然后编写如下程序：Sub S1(ByVal x As Integer, By Val y As Integer) Dim t As Integer t=x x=y y=tEnd SubPrivate Sub Command1_ Click() Dim a As Integer, b As Integer a=10 b=30 S1 a，b Print "a="；a="b="；bEnd Sub 程序运行后，单击命令按钮，输出结果是______。

A．a=30 b=10

B．a=30 b=30

C．a=10 b=30

D．a=10 b=10