用JS判断下拉框是否选中

<title>Js判断下拉框</title>
<script>
function ss(){
var slt=document.getElementById("aa");
if(slt.value==""){
alert("请选择一个项目");
return false;
}
return true;
}
</script>
<form method=post onsubmit=" return ss()">
<select id="aa">
<option value="">请选择</option>
<option value="1s">一</option>
<option value="2u">二</option>
</select>
<input type=submit value="提交">
</form>