js获取select标签选中的值

<p>
        城市：&nbsp;
        <select id="Select1" name="D1">
            <option value="1">北京</option>
            <option value="2">天津</option>
            <option value="3">湖南</option>
            <option value="4">新疆</option>
        </select>&nbsp;&nbsp;&nbsp;
        <input id="Button2" type="button" onclick="kk()" value="点击获得所选城市" />

</p>

<script type="text/javascript">

window.onload = function(){

function kk() {

var os = document.getElementById("Select1").value;

if(os=="1"){

alert("北京");

}

if(os=="2"){

alert("天津");

}

if(os=="3"){

alert("湖南");

}

if(os=="4"){

alert("新疆");

}

};

}

</script>