使用HTMLSELECT控件遇到的小麻烦，请高手指点

请各位高手比较以下两段代码:
这两段代码都是实现用:P_Symbol的选项"是"和"否"来决定控件"P_TYear"能否启用的功能
代码一纯粹为了实现该功能,代码二是在开发系统中使用.可是相同的代码在页面一中就能实现功能,但
在页面二就不能实现功能,为什么??
代码如下:
代码一(WEB页面一):
<%@ Page language="c#" Codebehind="WebForm4.aspx.cs" AutoEventWireup="false" Inherits="keyanchu.WebForm4" %>
<table height="36" cellSpacing="0" cellPadding="0" width="778" border="0">
<tr>
<td height=18>
<SELECT id="P_Symbol" onchange="fnChange(this.value)" name="P_Symbol" runat="server">
<option value="1" selected>是</option>
<option value="0">否</option>
</SELECT>
</td>
</tr>
<tr>
<td>
<select id="P_TYear" name="P_TYear" runat="server">
</select>
<SCRIPT language="JavaScript">
     function fnChange(value)
      {
        if(value=="1")
        { 
            P_TYear.disabled= false;
        }
else
{
            P_TYear.disabled= true;         
}
      }
</SCRIPT>
</td>
</tr>
</table>代码二(页面二):
<%@ Register TagPrefix="Acme" TagName="Footer" Src="Pagelet_Footer.ascx" %>
<%@ Register TagPrefix="Acme" TagName="Header" Src="Pagelet_Header.ascx" %>
<%@ Page language="c#" Codebehind="Project_Add.aspx.cs" AutoEventWireup="false" Inherits="keyanchu.Project_Add" %>
<%@ Import namespace="keyanchu" %>
<ACME:HEADER id="Header1" runat="server" Caption="项目管理"></ACME:HEADER>
<table height="495" cellSpacing="0" cellPadding="0" width="778" border="0">
  <tr>
    <td class="cVLine"></td>
    <td class="cNavier" vAlign="top" width="176">
       <table cellSpacing="0" cellPadding="0" width="100%">
<tr>
   <td class="cBand1">热点新闻</td>
</tr>
<tr>
   <td class="cHLine"><FONT face="宋体"></FONT></td>
</tr>
<tr>
   <td class="cBand1">纵向项目管理</td>
</tr>
<tr>
   <td>
      <table cellSpacing="0" cellPadding="0" width="100%">
<tr>
<td align="center"><br>
  ※<A href="Project_Add.aspx">纵向项目添加</A>※<br>
  ※<A href="Project_List.aspx">纵向项目浏览</A>※<br>
  ※<A href="ProjectOne_List.aspx">纵向合同浏览</A>※<br>
  ※<A href="Project_Query.aspx">纵向项目查询</A>※<br>
</td>
</tr>
      </table>
   </td>
</tr>
      </table>
   </td>
   <td class="cVLine"></td>
   <td class="cBody" vAlign="top" align="center" width="599">
      <div class="cBlock" style="WIDTH: 400px; HEIGHT: 450px">
      <form id="frmFileAdd" method="post" encType="multipart/form-data" runat="Server">
      <input id=PID type=hidden value='<%=Request["PID"]%>'>
      <table cellSpacing="1" cellPadding="1" width="300" align="center">
   <TBODY>
      <tr>
<td align="center" colSpan="2" height="30"><b><asp:label id="lbTitle" runat="Server">纵向项目添加</asp:label></b></td>
      </tr>
      <tr>
<td align="right" width="87">序号</td>
<td><asp:textbox id="P_SN" runat="server" size="31"></asp:textbox>
             <asp:requiredfieldvalidator id="P_RFVSN" runat="server" ControlToValidate="P_SN" Display="Dynamic" ErrorMessage="序号不能为空"></asp:requiredfieldvalidator>
             <asp:comparevalidator id="P_CVSN" runat="server" ControlToValidate="P_SN" Display="Dynamic" ErrorMessage="请输入整型数值"
Type="Integer" Operator="DataTypeCheck"></asp:comparevalidator>
         </td>
     </tr>
     <tr>
<td align="right" width="87">项目编号</td>
<td><asp:textbox id="P_Number" runat="server" size="31"></asp:textbox>
             <asp:requiredfieldvalidator id="P_RFVNumber" runat="server"  ControlToValidate="P_Number" Display="Dynamic" ErrorMessage="项目编号不能为空"></asp:requiredfieldvalidator></td>
    </tr>
    <tr>
<td align="right" width="87">项目名称</td>
<td><asp:textbox id="P_Name" runat="server" size="31"></asp:textbox></td>
    </tr>
    <tr>
<td align="right" width="87">任务来源</td>
<td><asp:textbox id="P_Mission" runat="server" size="31"></asp:textbox></td>
    </tr>
    <tr>
<td align="right" width="87">项目类别</td>
<td><asp:dropdownlist id="P_Type" runat="server"  width="136px"></asp:dropdownlist></td>
   </tr>
   <tr>
<td align="right" width="87">组织单位</td>
<td><asp:textbox id="P_Organize" runat="server" size="31"></asp:textbox></td>
   </tr>
   <tr>
         <td align="right" width="87" height="28">承担单位</td>
         <td height="28"><FONT face="宋体"><asp:dropdownlist id="P_Dept" runat="server" Width="206px"></asp:dropdownlist></FONT></td>
   </tr>
   <tr>
         <td align="right" width="87">项目负责人</td>
         <td><asp:textbox id="P_Person" runat="server" size="31"></asp:textbox></td>
   </tr>
   <tr>
<td align="right" width="87">签定日期</td>
         <td><asp:dropdownlist id="P_Year" runat="server"></asp:dropdownlist>年
<asp:dropdownlist id="P_Month" runat="server"></asp:dropdownlist>月
<asp:dropdownlist id="P_Day" runat="server"></asp:dropdownlist>日</td>
   </tr>
   <tr>
<td align="right" width="87">生效日期</td>
<td><asp:dropdownlist id="P_BYear" runat="server"></asp:dropdownlist>年
    <asp:dropdownlist id="P_BMonth" runat="server"></asp:dropdownlist>月
    <asp:dropdownlist id="P_BDay" runat="server"></asp:dropdownlist>日</td>
   </tr>
   <tr>
<td align="right" width="87" height="39">终止日期</td>
         <td height="39">
         <asp:dropdownlist id="P_EYear" runat="server"></asp:dropdownlist>年
<asp:dropdownlist id="P_EMonth" runat="server"></asp:dropdownlist>月
<asp:dropdownlist id="P_EDay" runat="server"></asp:dropdownlist>日</td>
   </tr>
   <tr>
<td align="right" width="87">合作单位</td>
<td><asp:textbox id="P_CooDept" runat="server" size="31"></asp:textbox></td>
   </tr>
   <tr>
<td align="right" width="87">计划经费</td>
<td><asp:textbox id="P_Money" runat="server" size="31"></asp:textbox>
             <asp:requiredfieldvalidator id="P_RFVMoney" runat="server" ControlToValidate="P_Money" Display="Dynamic" ErrorMessage="计划经费不能为空"></asp:requiredfieldvalidator>
             <asp:comparevalidator id="P_CVMoney" runat="server" ControlToValidate="P_Money" Display="Dynamic" ErrorMessage="请输入货币型数值"
Type="Currency" Operator="DataTypeCheck"></asp:comparevalidator>
         </td>
   </tr>
   <tr>
<td align="right" width="87" height="28">是否结题</td>
<td height="28"><select id="P_Symbol" onchange="fnChange(this.value)" name="P_Symbol" runat="server">
<option value="1" selected>是</option>
<option value="0">否</option>
</select>
</td>
   </tr>
   <TR>
<TD align="right" width="87" height="25">是否归档</TD>
<TD height="25">
             <asp:dropdownlist id="P_End" runat="server">
       <asp:ListItem Value="否">否</asp:ListItem>
       <asp:ListItem Value="是">是</asp:ListItem>
    </asp:dropdownlist></TD>
   </TR>
   <TR>
<TD align="right" width="87" height="45">实际结题日期</TD>
<TD height="28"><select id="P_TYear" name="P_TYear" runat="server">
     <option value="1978" selected>1978</option>
</select>
<SCRIPT language="JavaScript">
     function fnChange(value)
     {
if(value=="1")
 { 
    P_TYear.disabled= false;
 }
else
{
    P_TYear.disabled= true;         
}
      }
</SCRIPT>
          年
    <asp:dropdownlist id="P_TMonth" runat="server"></asp:dropdownlist>月
    <asp:dropdownlist id="P_TDay" runat="server"></asp:dropdownlist>日</TD>
    </TR>
    <TR>
<TD vAlign="top" align="right" width="87">进展情况说明</TD>
<TD><asp:textbox id="P_Re" runat="server" cols="24" rows="14" TextMode="MultiLine"></asp:textbox></TD>
    </TR>
    <tr>
<td align="center" colSpan="2" height="40">
            <asp:button id="btnSave" onclick="btnSave_Click" runat="server" Text=" 保 存 " cssclass="cBtn">
             </asp:button><input class="cBtn" type="reset" value=" 重 添 "></td>
    </tr>
    </TBODY>
   </table>
 </form>
</div>
</td>
<td class="cVLine"></td>
</tr>
</table>
<ACME:FOOTER id="Footer1" runat="server"></ACME:FOOTER>