Asp.net C# hide TR

<table>
        <tr>
            <td>Type</td>
            <td>
                <asp:DropDownList ID="typeDDL" runat="server" AutoPostBack="true">
                    <asp:ListItem Text="Please select" Value="" Selected="True"></asp:ListItem>
                    <asp:ListItem Text="Electricity" Value="Electricity"></asp:ListItem>
                    <asp:ListItem Text="Gas" Value="Gas"></asp:ListItem>


                </asp:DropDownList></td>
        </tr>
        <tr  runat="server" visible="false" id="mmbtu_convtr">
            <td>MMBTU_CONV</td>
            <td>
                <input type="text" runat="server" id="mmbtu_conv" name="mmbtu_conv" value="" style="width: 250px" /></td>
        </tr>
</table>

protected void Page_Load(object sender, EventArgs e)
        {
            if (typeDDL.SelectedValue == "gas")
            {
                mmbtu_convtr.Visible = true;
                pkstr.Visible = true;
            }
            else
            {
                mmbtu_convtr.Visible = false;
                pkstr.Visible = false;

            }
        }