radio被选中，但是重复点击后事件不触发

$(‘.ss‘).click(function(){
               $(this).find("input[type=radio]").attr("checked",true);
   });

<ul>
  <li class="ss"><input type="radio" name="radio" class="fc" id="radio" value="a" />fdfd</li>
  <li class="ss"><input type="radio" name="radio" class="fc" id="radio" value="b" />qqqq</li>
  <li class="ss"><input type="radio" name="radio" class="fc" id="radio" value="c" />wwww</li>
  </ul>

$(‘.ss‘).click(function(){
    var    $radio = $(this).find("input[type=radio]"),
           $flag  = $radio.is(":checked");
    if( !$flag ){
        $radio.prop("checked",true);   
    }
});