Why My RadioButton in the GridView Can't be unchecked

I am using VB.NET
I am using RadioButton control using the <TemplateField> inside the GridView. The problem is that the RadioButton doesnt behave correctly. When another record checked, the previous is still checked where it supposed to be unchecked.
I would like to see how to make it instead of seeing material links as I read alot but I have difficulty in understanding.
The following is my markup view. Actually I don't know what I should write in the CodeBehind.
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="SqlDataSource1" style="margin-right: 9px; margin-top: 30px" Width="910px" Height="100px" CellPadding="4" ForeColor="#333333" GridLines="Vertical" BackColor="Black" BorderStyle=" ...