title: Jquery hide/show optgroup base on main selector
body: <p>Please refer to this <a href="http://jsfiddle.net/wnFXX/7/" rel="nofollow">link</a> , is a multiple select box to select sub-activity base on what is picked in main-activity, its working pretty well now. </p>
  
  <p>Based on change request, I want to make the action to <strong>show only</strong> the optgroup in sub-activity when it being select, means when you select <code>main-activity option 1 and 2</code>, <code>sub-activity</code> are show optgroup for <code>1 and 2</code>, the rest are hide and not visible, please advise, thanks.</p>