回文字符串的特征是从左到右读和从右到左读是完全一样的，如“aba”、“aabbaa”，判断字符串中忽略“，”字符后，该字符串是否是回文字符串的VB程序段如下：

s = "ab,,,cb,a"

n = Len(s)

i = 1: j = Len(s)

flag = True

Do While i <= j

    If Mid(s, i, 1) = Mid(s, j, 1) Then

        i = i + 1: j = j - 1

    Else

        

    End If

Loop

If flag = False Then Print ("不是回文") Else Print ("回文")

方框中的代码由以下三部分组成：

① If Mid(s, i, 1) = "," Or Mid(s, i, 1) = "." Then i = i + 1

② If Mid(s, j, 1) = "," Or Mid(s, j, 1) = "." Then j = j – 1

③ If Mid(s, i, 1) <> "," And Mid(s, j, 1) <> "," Then flag = False: Exit Do

下列选项中，代码顺序正确的是：（     ）

A、①②③

B、②①③

C、②③①

D、③②①