在窗体上画一个命令按钮，然后编写如下程序： Sub S1(By Val x As Integer, By Val y As Integer) Dim t As Integer t=x x=y y=r End Sub Private Sub Command1_Click() Dim a As Integer, b As Integer a=10 b=30 S1 a，b Print'a=';'b=';b End 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

A.输出结果是______。
A.a=30
B.a=30
C.a=10
D.a=10