title: switching css class on multiple divs
body: <pre><code>&lt;div class='checked' id='rb01'&gt;&lt;/div&gt;
  &lt;div class='rb' id='rb02'&gt;&lt;/div&gt;
  &lt;div class='rb' id='rb03'&gt;&lt;/div&gt;
  </code></pre>
  
  <p>these are buttons. clicking I need to switch css class.</p>
  
  <pre><code>$(".rb").click(function() {
      $('.checked').addClass('rb');  // doesn't work
      $(this).addClass('checked');  // works
  });
  $("#rb01").click(function() {  // all works
      $('#player').attr('src', '//www.youtube.com/embed/x13hzEsEPe0?rel=0');
  });
  $("#rb02").click(function() {
      $('#player').attr('src', '//www.youtube.com/embed//pKfRAq4PPIQ?rel=0');
  });
  $("#rb03").click(function() {
      $('#player').attr('src', '//www.youtube.com/embed/hiZIEB1776g?rel=0');
  });
  </code></pre>