Select one dropdown value bind the values to combo box and select another dropdown value bind the values to same combo box but previous values are no changes ....?

<form id="form1" runat="server">
<div>
    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"
        onselectedindexchanged="DropDownList1_SelectedIndexChanged">
        <asp:ListItem>Akil</asp:ListItem>
        <asp:ListItem>Arun</asp:ListItem>
        <asp:ListItem>Balu</asp:ListItem>
        <asp:ListItem></asp:ListItem>
    </asp:DropDownList>
    <br />
    <br />
    <br />
    <asp:ComboBox ID="ComboBox1" runat="server" >

    </asp:ComboBox>
</div>
</form>

int i = 0;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        ComboBox1.Items.Insert(i,DropDownList1.SelectedItem.Text);
        i++;
    }