在html的form中的下拉菜单里使用JavaScript实现弹窗

请问如何在html中的form里实现：单击选中下拉菜单中的一项后弹出风险提示呢，试过了链接javascript但没反应，谢谢各位了～

 <div id="select">
                <select name="select" placeholder="Please choose the size of the burger" id="select">
                    <option>Please choose the size of the burgeroption>
                    <option>6oz Larger  7.00 eurooption>
                    <option>10oz Extra-Larger   9.00 eurooption>
                    **<option onclick="a_function()">16oz Supersize  17.50 eurooption>**
                select>
            div>

function a_function(){
    alert("Supersize burger is super huge, please confirm whether to continue");
}

大致是这个样子，在这个下拉菜单中实现单击16oz后弹窗，谢谢啦！

<div id="select">
                <select name="select" placeholder="Please choose the size of the burger" onclick="a_function()" id="selectss">
                    <option  value="1" >Please choose the size of the burger</option>
                    <option value="2" >6oz Larger  7.00 euro</option>
                    <option value="3" >10oz Extra-Larger   9.00 euro</option>
                    **<option value="4" >16oz Supersize  17.50 euro</option>**
                </select>
            </div>
            <script>
            function a_function(){
var obj=document.getElementById("selectss"); 
 if(obj.value=='4'){
alert("Supersize burger is super huge, please confirm whether to continue");}
    
}
            </script>