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加密，是以某种特殊的算法改变原有的信息数据。某一字符串的加密算法为：
（1）将明文中每个字符的八位二进制ASCII码（不足8位左端补0，凑足8位）分成2段，左4位为高位段，右4位为低位段，如字符“C”的二进制ASCII码值为01000011，分段后0100和0011；
（2）将高位段左移一位，将原第一位数移至第4位（如0100转化为1000），再将其转化为十六进制数（如1000转化为8）；
（3）对低位段执行（2）相同的算法，如001101106
（4）顺次连接两位十六进制数，得到该字符的密文，如“C”的密文为“86”；
（5）将每个字符的密文按照明文的顺序连接。
小方按照上述方法，设计一个字符串解密的VB程序，功能如下：在文本框Text1中输入密文，单击“解密”按钮Command1，程序依次将密文中每两个字符转换为一个明文字符，并将解密结果输出在文本框Text2中。程序运行效果如图所示。

（1）按照解密方法，密文“88”对应的明文字符是___________。
（2）实现上述功能的VB程序如下，请在划线处填入合适代码。
             Private Sub Command1_Click()
                    Dim a(15) As String
                    Dim pwd As String, i As Integer
                    Dim p1 As String, p2 As String
                    Dim st As String, pla As String
                    a(0) = "0000": a(1) = "0001": a(2) = "0010": a(3) = "0011"
                    a(4) = "0100": a(5) = "0101": a(6) = "0110": a(7) = "0111"
                    a(8) = "1000": a(9) = "1001": a(10) = "1010": a(11) = "1011"
                    a(12) = "1100": a(13) = "1101": a(14) = "1110": a(15) = "1111"
                    pwd = Text1.Text
                    st = "": pla = ""
                    __________①          
                    Do While i <= Len(pwd)
                           p1 = Mid(pwd, i - 1, 1)
                           p2 = Mid(pwd, i, 1)
                           ______________②               
                           st = Mid(st, 4, 1) + Mid(st, 1, 3) + Mid(st, 8, 1) + Mid(st, 5, 3)
                           pla = pla + Chr(btod(st))
                           i = i + 2
                    Loop
                    Text2.Text = pla
             End Sub
             Function seat(x As String) As Integer
                    If x >= "A" And x <= "F" Then
                           seat = Asc(x) - Asc("A") + 10
                    Else
                           seat = Val(x)
                    End If
             End Function
             Function btod(y As String) As Integer     '将8位二进制数转换为十进制数
                    Dim da As Integer, i As Integer, ch As String
                    da = 0
                    For i = 1 To 8
                           ch = Mid(y, i, 1)
                           _____________③                
                    Next i
                    btod = da
             End Function