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编写VB程序，实现如下功能：在文本框Text1 输入字母字符串，单击“加密”按钮Command1，字符进行逐个加密，加密过程是，先在“密码本”m中找到相应字符，再从Text3中得到其所对应位置的密钥,并在文本框Text2中显示密文（运行效果如下图所示）。本题暂时不考虑解密问题。实现上述功能的VB代码如下：

请在划线处填入合适代码。
Private SubCommand1_Click()
Dim s As String, m As String
Dim t As String
Dim k As String
Dim c As Integer
s = Text1.Text
n1 = Len(s)
s3 = Text3.Text
n3 = Len(s3)
If n3 < 26 Then
Label1.Caption = "请重新输入密钥！"
Text3.Text = ""
Exit Sub
End If
m = "abcdefghijklmnopqrstuvwxyz"
For i =" 1" To n1
k =" Mid(s," i, 1)
If ①Then '将大写字母转换为小写
k =" Chr(Asc(k)" + 32)
End If
c = 1
Do While c <= Len(m)
If k =" Mid(m," c, 1) Then
Exit Do
End If
②
Loop
t =" t" +③
Next i
Text2.Text = t
End Sub