关于连动菜单的问题

<script>
var arr1=["a","b","c"]
var arr2=["d","e","f"]
var arr3=["g","h","i"]
function fill(obj,arr){
obj.length=0;
for(i=0;i<arr.length;i++)obj.options[obj.length++].text=arr[i]
}
</script>
<input type=radio name=s onclick=fill(s1,arr1)>1
<input type=radio name=s onclick=fill(s1,arr2)>2
<input type=radio name=s onclick=fill(s1,arr3)>3
<select name=s1></select>