怎么在javascript获得表单中单选按钮的值

如何在javascript获得表单中单选按钮的值
如何在javascript获得表单中单选按钮的值并判断选择的是哪一个，请大家帮帮忙，谢谢

[解决办法]
getElementById
getElementsByName
getElementsByTagName

判断 checked 属性
[解决办法]

HTML code

<form onsubmit='return check()'><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='checkbox' name='cb'/><input type='submit'/></form><script type='text/javascript'>function check(o){    var msg = "";    var checkboxes = document.getElementsByName('cb');    for(var i = 0 ; i < checkboxes.length ; i ++)    {        if(checkboxes[i].checked) msg +='第'+(i + 1) + "个被选中\n";    }    alert(msg)}</script>