Javascript Radio Button Validation

<script type="text/javascript">
    function validate()
    {
        if (!checkRadio("frm1","right_answer"))
          alert("You didnt select any answer");
        else
          alert("You selected a answer");
    }
    function checkRadio (frmName, rbGroupName)
    {
       var radios = document[frmName].elements[rbGroupName];
       for (var i=0; i <radios.length; i++)
      {
        if (radios[i].checked)
       {
        return true;
        }
      }
     return false;
    }
</script>

<form name="frm1" method="post" action="#">
    <table >
        <tr><td><input type="radio" name="r_answer" id="r_answer" value="answer1" />Answer 1</td></tr>
        <tr><td><input type="radio" name="r_answer" id="r_answer" value="answer2" />Answer 2</td></tr>
        <tr><td><input type="radio" name="r_answer" id="r_answer" value="answer3" />Answer 3</td></tr>
        <tr><td><input type="submit" name="Ok" value="Ok" onclick="return validate();"/></tr>
    </table>
</form>