var how_many_ads = 3; 
var now = new Date() 
var sec = now.getSeconds() 
var ad = sec % how_many_ads; 
ad +=1; 
if (ad==3) { 
url="/team.asp"; 
alt="欢迎你的加入！一起开始，其实不难。"; 
banner="/green/jiaodian/zhm.gif"; 
width="518"; 
height="60";
} 
if (ad==1) { 
url="/register.asp"; 
alt="现在就加入绿行者行列，每周关注！"; 
banner="/green/jiaodian/guanzhu.jpg"; 
width="518"; 
height="60";
} 
if (ad==4) { 
url="http://baike.baidu.com/view/10754.htm"; 
alt="历史不能忘记！"; 
banner="/green/jiaodian/77.jpg"; 
width="518"; 
height="60";
} 
if (ad==2) { 
url="/register.asp"; 
alt="厘清纷杂，提取精华"; 
banner="/green/jiaodian/gaoduan.gif"; 
width="518"; 
height="60";
} 
document.write('<center>'); 
document.write('<a href="' + url + '" target="_top">'); 
document.write('<img src="' + banner + '" width=') 
document.write(width + ' height=' + height + ' '); 
document.write('alt="' + alt + '" border=0>'); 
document.write('</a></center>');