radio 刷新,怪问题~~~~~~~

<html><head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>新建网页 2</title>
</head><body>
A:<input name="a" id="a" value="1" type=radio>
B:<input name="a" id="a" value="2" type=radio>
</body>
<script>
document.all["a"][0].checked=true;
</script>
</html>我想每次刷新,都默认选中A.
但我一选中B之后,然后无论怎样刷新,都是选中B.
为什么????