button按钮点击后过段时间不能点击-验证码(刷新后失效)

<html>

<head>
<script language="JavaScript">
function dd(){
//提交操作
var btn = document.getElementById("btn")
btn.disabled=true
c(60)
}
function c(i){
var btn = document.getElementById("btn")
i--;
if(i==0){
btn.value = "确定"
btn.disabled=false
}
else{
btn.value = "等待("+i+")秒后可以重新操作"
setTimeout("c("+i+")",1000)
}
}
</script>
</head>