这个错误如何解决？

错误：“HtmlSelect”不能有类型为“LiteralControl”的子级。
<select id="DDLYear" onchange="toDate()" style="width: 71px" runat="server">

<script>for(i=1970;i<=2010;i++)document.write("<option>"+i+"</option>")</script>

</select>
&nbsp;年&nbsp;
<select id="DDLMonth" onchange="toDate()" style="width: 78px" runat="server">

<script>for(i=1;i<=12;i++)document.write("<option>"+i+"</option>")</script>

</select>
&nbsp;月&nbsp;
<select id="DDLDay" onchange="toDay()" style="width: 71px" runat="server">
</select>