大家帮帮我这菜鸟吧！
<html><head>
<title>综合练习</title>
<script>
function jieshou()
{
document.list.tt.value=document.list.book.value + " 接收焦点";
}
function sum()
{
var i;
var s="";
for{i=0;i<document.list.book.length;i++}{
if(document.list.book[i].checked==true)
{
s=s+document.list.book[i].value;
}
x=1;
}
if(x==1)
{
document.write("<h2>以下是您订阅的时事通讯</h2>")<br>;
document.write(s);
}
else
alert("您没有选择时事通讯！");
}
</script>
</head>
<body>
<p> 欢迎订阅时事通讯</p>
请选择您要订阅的时事通讯：
<form name="list" action="" method="post">
<input type="checkbox" name="book" value="C" onFocus="jieshou()">C<BR>
<input type="checkbox" name="book" value="C++" onFocus="jieshou(">C++<BR>
<input type="checkbox" name="book" value="C#" onFocus="jieshou()">C#<BR>
<input type="checkbox" name="book" value="Visual Basic" onFocus="jieshou()">Visual Basic<BR>
<input type="checkbox" name="book" value="Oracle" onFocus="jieshou()">Oracle<BR>
<input type="checkbox" name="book" value="SQL Server" onFocus="jieshou()">SQL Server<BR>
<input type="text" name="tt"><BR>
<input type="button" value="登记" onClick="sum()">
</form>
</body>
</html>
大家帮我看看哪里出了问题！！！急啊，星期六要交作业！！
