function show_intro(pre,pree, n, select_n,css) {
  for (i = 1; i <= n; i++) {
    var intro = document.getElementById(pre + i);
    var cha = document.getElementById(pree + i);
    intro.style.display = "none";
    cha.className=css + "_off";
    if (i == select_n) {
      intro.style.display = "block";
      cha.className=css + "_on";
    }else{
      if (i ==(select_n-1)){
        cha.className=css + "_of";
      }else{
        cha.className=css + "_off";
      }
    }
  }
}