How to make the selected index change without refreshing the page

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C#

Hi,

In my application I am using the dropdownlist for that I have return the selected index change event for making some controls to visible true and false. I have used the update panel for not refreshing the whole page. But when I select the dropdown the page is refreshing for every time. In my application I have used 7 updatepanels.
Can anyone solve my problem. Here is the code:

<tr>
       <td>
        <asp:UpdatePanel id="UpdatePanel1" runat="server" UpdateMode="Conditional">
    <contenttemplate>
       <table align="center">




                <tr>

    <td align="left" >
        <asp:Label id="lblPriorityId" runat="server" CssClass="css_normalLabel"
            Text="Priority:" ></asp:Label></td>
    <td align="left" >
                <asp:DropDownList ID="ddlPriorityId" runat="server" CssClass="css_dropdown"
                    TabIndex="27" Width="100px">
                    <asp:ListItem value="S">Standard</asp:ListItem>
                    <asp:ListItem value="H">High</asp:ListItem>
                </asp:DropDownList></td>
                <td align="right">
            <asp:Label id="lblDecideToBidId" runat="server" CssClass="css_normalLabel"
            Text="Bid Manager Decision:" ></asp:Label></td>
                    <td align="left"
                        >
                <asp:DropDownList ID="ddlDecideToBidId" runat="server" CssClass="css_dropdown"
                            TabIndex="28" AutoPostBack="true"  OnSelectedIndexChanged="ddlDecideToBidId_SelectedIndexChanged"
                            Width="100px">

                    <asp:ListItem value="Y">Go</asp:ListItem>
                    <asp:ListItem value="N">No Go</asp:ListItem>
                </asp:DropDownList>

        </td></tr>

        <tr><td align="left">
<asp:Label ID="lblBidOwner" CssClass="css_normalLabel"  runat="server"
         Text="Bid Owner:"></asp:Label>
        </td><td align="left" >
            <asp:RadioButtonList ID="rdbBidowner" runat="server" CssClass="css_radiobutton_list"
                OnSelectedIndexChanged="rdbBidOwner_SelectedIndexChanged"
                RepeatDirection="Horizontal" AutoPostBack="True" Width="150px" TabIndex="29" >
                <asp:ListItem Value="S">Self</asp:ListItem>
                <asp:ListItem Value="B">Bid Engineer</asp:ListItem>
            </asp:RadioButtonList>
            </td>
            <td align="left">
             <asp:DropDownList ID="drpBidEngineer"  runat="server"
                 CssClass="css_dropdown" Visible="false" TabIndex="30" Width="100px">
            </asp:DropDownList>
        </td>
        </tr>


        </table>
        </contenttemplate>
       </asp:UpdatePanel>
       </td>
       </tr>

Thanks in Advance