用JS判断单选按钮是否选中

<script>
function check(){
var flag = false;
if(form1.info_type != null){
var msglen=form1.info_type.length;
for(i=0;i<msglen;i++){
if(form1.info_type[i].checked==true){
flag = true;
}
}
}
if (flag == false)
{
alert("对不起，请选择")
return false;
}
}
</script>

<form id="form1" name="form1" method="post" action="" onsubmit="return check();">
<td height=30>A
<input type="radio" name="info_type" value="A">
B
<input type="radio" name="info_type" value="B">
C
<input type="radio" name="info_type" value="C">
<label>
<input type="submit" name="Submit" value="提交" />
</label></td>
</form>