对称字符串从左读到右和从右读到左完全一样,如“1221”“12321”。编写VB程序,判断输入字符串是否对称,程序运行界面如图a所示,在文本框Text1中输入字符串,单击Command1按钮,在标签Label2上显示判断结果。

图a

图b
(1)实现上述功能的VB程序如下,请在画线处填入合适的代码。
Private Sub Command1_Click()
Dim s As String, c1 As String, c2 As Integer
Dim n As Integer, i As Integer
Dim f As Boolean
s = Text1.Text
n = Len(s)
f = True
For i = 1 To n \ 2
c1 =
_______①______ c2 = Mid(s, n - i + 1, 1)
If c1 <> c2 Then
f = False
Exit For
End If
Next i
If
② Then
Label2.Caption = "对称"
Else
Label2.Caption = "不对称"
End If
End Sub
划线处代码应为
①________________
②________________
(2)运行该程序,在文本框中输入aba,单击“判断”按钮后,出现图b所示错误提示,引起该错误的原因是________________(单选,填字母:
A.变量s的数据类型错误; | B.变量c2的数据类型错误; | C.变量f的数据类型错误)。 |