js动态加载DropDownList，清空怎么实现

js动态加载DropDownList，清空如何实现
我想在执行  switch之前将DropDownList清空

<asp:TableRow Height="40">
          <asp:TableCell ID="TableCell7" runat="server"  CssClass="tb_edit_text"  BackColor="#CFDEE9">
             <asp:Label ID="Label8" runat="server" Width="90px" Text="选择"  CssClass="tb_edit_label" />
             <asp:DropDownList runat="server" ID="yslx">
               
             </asp:DropDownList>
            <asp:Label ID="biaoti" runat="server" style="color:Red" CssClass="tb_edit_label" Text=""  />  
          </asp:TableCell> 
         
        </asp:TableRow>
<script type="text/javascript">
 switch(d)
   　　      {
　　          case 1:
 　　            document.all("yslx").add(new Option("1","1"));
 　　            break
　　         case 2:
　　             document.all("yslx").add(new Option("2","2"));
　　            break
......　　        　
 } 
[解决办法]
document.getElementById("<%=yslx.ClientID %>").removeChild(document .getElementById ("<%=yslx.ClientID %>").firstChild )