下拉列表如何互相控制？

Question

<select id="firstList" onchange="javascript:switchSelect()">
<option value="1" selected>开</option>
<option value="0">关</option>
</select>
<select id="secondList">
<option></option>
</select>
<script language="javascript">
function switchSelect()
{
var source=document.getElementById("firstList");
var target=document.getElementById("secondList");
target.options[0].value=source.options[source.selectedIndex].value;
target.options[0].text=source.options[source.selectedIndex].value;
}

window.onload=switchSelect;
</script>