单选题编写如下事件过程：
Private sub sub1 (ByVal x1 As String, y1 As String)
Dim xt As String
Dim i As Integer
i = Len(x1)
Do While i＞= 1
xt = xt + Mid(x1, i, 1)
i=i-1
Loop
y1 = xt
End Sub
Private Sub Form Click()
Dim s1 As String, s2 As String
s1= "teacher"
sub1 s1, s2
Print s2
End Sub
程序运行后，单击窗体，则窗体上显示的内容是

A、 rehcaet
B、 tahreee
C、 themee
D、 eerthea

【正确答案】 A

【答案解析】[分析] 可以看出，Sub1(ByVal x1 As String，y1 As String)的作用是将参数x1的字符逆序处理，然后赋给y1，而y1是传址引用的，可以直接返回处理结果。