简单的asp生成验证码

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<%
Option Explicit
Response.buffer=true
NumCode
Function NumCode()
    Response.Expires = -1
    Response.AddHeader "Pragma","no-cache"
    Response.AddHeader "Cache-Control","no-cache"
    On Error Resume Next
    Dim zNum,i,j
    Dim Ados,Ados1
    Randomize timer
    zNum = cint(8999*Rnd+1000)
    Session("image_check") = zNum
    Dim zimg(4),NStr
    NStr=cstr(zNum)
    For i=0 To 3
        zimg(i)=cint(mid(NStr,i+1,1))
    Next
    Dim Pos
    Set Ados=Server.CreateObject("Adodb.Stream")
    Ados.Mode=3
    Ados.Type=1
    Ados.Open
    Set Ados1=Server.CreateObject("Adodb.Stream")
    Ados1.Mode=3
    Ados1.Type=1
    Ados1.Open
    Ados.LoadFromFile(Server.mappath("image_code_body.Fix"))