Hide and show option

nav.js

$ = jQuery;
  $(document).ready(function($){
    var nowShowing = $(".selected");
    var showHandler = function(event) {
      event.preventDefault();
      var t = $(event.target);
      if (nowShowing) {
        nowShowing.slideUp("slow");
      }
      nowShowing = t.parent().next();
      nowShowing.slideDown("slow");
      return false;
    };
    $("dd").hide();
    nowShowing = $(".selected");
    if (nowShowing) {
      nowShowing.show();
    }
    $("dt a").click(showHandler);
    $("#top_nav").show();
  });