$(function () {
  var nav = $("#c_navigation_029_P_019-1669107905653"); 
  var win = $(window); //寰楀埌绐楀彛瀵硅薄  
  var sc = $(document); //寰楀埌document鏂囨。瀵硅薄銆  
  win.scroll(function () {
    if (sc.scrollTop() >= 20) {
      nav.addClass("sc_active");
    } else {
      nav.removeClass("sc_active");
    }
  })
})