<input type="button" id="btn" value="http://www.mamicode.com/按钮"/>
<select name="select" id="select">
    <option value="http://www.mamicode.com/1">aa</option>
    <option vhttp://www.mamicode.com/alue="a">bb</option>
    <option value="http://www.mamicode.com/c">cc</option>
</select>
<script type="text/javascript">
//点击按钮，让第二项选中
    document.getElementById(‘btn‘).onclick=function(){
        document.getElementById(‘select‘).vhttp://www.mamicode.com/alue="a";
    }    
</script>