有如下VB程序段：

s = Text1.Text

For i = 1 To Len(s)

    c = Mid(s, i, 1)

    If c >= "A" And c <= "Z" Then c =      ①     

    If c >= "a" And c <= "z" Then

        m =       ②       

        c = Chr(Asc("a") + m)

    End If

    s1 = s1 + c

Next i

Label1.Caption = s1

程序运行时，在文本框Text1中输入“ABC123xyz”，在标签框Label1中输出“bcd123yza”，则划线处的代码为              

A、① Chr(Asc(c) + 32)    ②(Asc(c) - Asc("a")) Mod 26 + 1

B、① Chr(Asc(c) + 32)     ②(Asc(c) - Asc("a") + 1) Mod 26

C、① Chr(Asc(c) - 32)     ②(Asc(c) - Asc("a")) Mod 26 + 1

D、① Chr(Asc(c) - 32)     ②(Asc(c) - Asc("a") + 1) Mod 26