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