做一个问题，如果输入的答案正确则弹出正确，错误弹出错误

HTML代码：

 1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 2 <html xmlns="http://www.w3.org/1999/xhtml">
 3 <head>
 4 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 5 <title>无标题文档</title>
 6 </head>
 7 
 8 <body>
 9 <form>中华民国成立于哪一年<input type="text" daan="1912" value="" id="t1" name="t1" />
10 <input type="button" onclick="check()" id="t2" name="t2" value="检查答案" />
11 </form>
12 </body>
13 </html>
14 <script>
15 function check()
16 {
17 var a=document.getElementById("t1");
18 var a1=a.value;
19 var a2=a.getAttribute("daan");
20 if(a1==a2)
21 {
22     alert("恭喜你答对了");
23     }
24 else
25 {alert("笨蛋")}
26 }
27 </script>

效果为：