利用js给select动态添加option并选中该option

<script>function addOptionToSelect(){   var slt=document.getElementById("slt");   var objOption=document.createElement("OPTION");   objOption.value='6';   objOption.text='content';   slt.add(objOption);   alert(slt.options.length);   slt.options[slt.options.length-1].selected='selected';}function getvalue(){   var slt=document.getElementById("slt");   alert(slt.value);}</script><input type="button" value="add option" onclick="addOptionToSelect()"><br><select id="slt"><option value='1'>aaaa</option><option value='2'>bbbb</option><option value='3'>cccc</option><option value='4'>dddd</option><option value='5'>eeee</option></select><br><input type="button" value="get value" onclick="getvalue()">

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