如何能清除radio button 得值

<BODY>
<input type="radio" name="a" value="a" checked>
<input type="radio" name="a" value="b">
<input type="button" name="btn" value="confirm" onclick="clearStatusOfRadio()">
</BODY><script>
function clearStatusOfRadio(){
for(i=0;i<document.getElementsByName("a").length;i++)
document.getElementsByName("a")[i].checked=false
}
</script>