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";
            }
         }
}