单项选择题

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=t
End Sub
Private Sub Command1_ Click()
Dim a As Integer, b As Integer
a=10
b=30
S1 a,b
Print "a=";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