// JavaScript Document 
    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"; 
            } 
        } 
    }