在ASP.NET下实现数字和字符相混合的验证码

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        <td colspan="2" valign="middle">验证码:</td>
        <td valign="top"><asp:Image id="Image1" runat="server" ImageUrl="gif.aspx" /></td>
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        <td colspan="2" valign="top">输入验证码:</td>
        <td valign="top"><asp:textbox ID="txtValidateCode" runat="server" TextMode="SingleLine" /></td>
        <td colspan="2" valign="middle"><font color="#FF0000" size="2">*注意:区分大小写</font></td>
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        <td valign="top"><asp:button ID="btnSubmit" runat="server" Text="比较" onclick="btnSubmit_click" /></td>
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