Asp.net radio buttons

if you have a page thus

      <p>

          <asp:RadioButton GroupName ="tt" ID="RadioButton1" runat="server"  Checked="true" />

            <asp:RadioButton  GroupName ="tt" ID="RadioButton2" runat="server" />

              <asp:RadioButton  GroupName ="tt" ID="RadioButton3" runat="server" />

      </p>

      <asp:Button ID="Button7" runat="server" Text="radio"

          onclick="Button7_Click" />

and code thus:

   protected void Button7_Click(object sender, EventArgs e)

        {

           // this.RadioButton2.Checked = false;

            this.RadioButton1.Checked = true;

        }

and you you select the 2nd radio button and click button7 - what will happen?

since the values are set on load - the button code will do nothing because you can't have to radio buttons selected.