function navigate(obj) {
  document.getElementById("nav_1").className = 'aa';
  document.getElementById("nav_2").className = 'aa';
  document.getElementById("nav_3").className = 'aa';
  document.getElementById("nav_4").className = 'aa';
  document.getElementById("nav_5").className = 'aa';
  document.getElementById("nav_6").className = 'aa';
 document.getElementById("nav_7").className = 'aa';
  document.getElementById(obj).className = 'active';
}