Populate Dropdown on selecetd index cahnge

cannot
clear the Dropdown items on selected index changed

the items get added one below the other

protected void ddlSection_SelectedIndexChanged(object sender, EventArgs e)
    {

         ddlSegement.ClearSelection();
        if (ddlSection.SelectedIndex == 1)
        {
              ddlSegement.Items.Add("Equity");
            ddlSegement.Items.Add("Derivatives");
              ddlSegement.Items.Add("Mutual Funds");
              ddlSegement.Items.Add("FII's");
            
        }

 
    if (ddlSection.SelectedIndex == 2)
    {
       
        ddlSegement.Items.Add("Companies/ Securities");
        ddlSegement.Items.Add("Capital listed");
        ddlSegement.Items.Add("Amount offered");
        ddlSegement.Items.Add("Indices");
        ddlSegement.Items.Add("Ratios");
        


    
    }

and ASpx page code is
<tr><td >Select Section</td><td colspan="3"><asp:DropDownList ID="ddlSection" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlSection_SelectedIndexChanged" >
<asp:ListItem Value="0" Selected="True" >Select</asp:ListItem>
<asp:ListItem Value="1" >Turnover Satistics</asp:ListItem>
<asp:ListItem Value="2">Listing and Captial</asp:ListItem>
<asp:ListItem Value="3">Membership and TWS</asp:ListItem>
<asp:ListItem Value="4">Client Statistics</asp:ListItem>


</asp:DropDownList></td></tr>

<tr ><td >Select Segement</td><td colspan="3"><asp:DropDownList ID="ddlSegement" runat="server" AutoPostBack="true" >

</asp:DropDownList></td></tr>