How to pass the gridview selected row's value to another page within texbox...

<asp:GridView ID="GridView1" runat="server" AllowPaging="True" PageSize="5"
       AutoGenerateColumns="False" BackColor="White" BorderColor="White"
       BorderStyle="Ridge" BorderWidth="2px" CellPadding="3" CellSpacing="1"
       DataKeyNames="PostDoubtID" DataSourceID="SqlDataSource1"
   GridLines="None" Width="918px">
       <Columns>
           <asp:CommandField ShowDeleteButton="True" />
           <asp:BoundField DataField="Topic" HeaderText="Topic" SortExpression="Topic" />
           <asp:BoundField DataField="Subject" HeaderText="Subject"
               SortExpression="Subject" />
           <asp:BoundField DataField="Date" HeaderText="Date" SortExpression="Date" />
           <asp:TemplateField HeaderText="Doubts In Detail">
               <ItemTemplate>
                   <asp:TextBox ID="TextBox1" runat="server" Height="76px"
                       Text='<%# Bind("YourDoubt") %>' TextMode="MultiLine" Width="400px"></asp:TextBox>
               </ItemTemplate>
           </asp:TemplateField>
           <asp:TemplateField HeaderText="Reply">
               <ItemTemplate>
                  <asp:LinkButton ID="LinkButton" runat="server"
                       PostBackUrl='<%# "~/Staff/Reply Doubts.aspx?id="+ Eval("Topic") %>' Text="Reply"></asp:LinkButton>
               </ItemTemplate>
           </asp:TemplateField>
       </Columns>