1.运行效果如图所示


2.实现代码如下

<!DOCTYPEhtml><html><head><metacharset="utf-8"><metahttp-equiv="X-UA-Compatible"content="IE=edge"><title>表单控件——下拉选择框select元</title><!--最新版本的Bootstrap核心CSS文件--><linkrel="stylesheet"href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css"integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh5u"crossorigin="anonymous"><!--可选的Bootstrap主题文件（一般不用引入）--><linkrel="stylesheet"href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap-theme.min.css"integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp"crossorigin="anonymous"></head><body><formrole="form"><!--单选--><divclass="form-group"><selectname=""id=""class="form-control"><optionvalue="">1</option><optionvalue="">2</option><optionvalue="">3</option><optionvalue="">4</option><optionvalue="">5</option></select></div><!--多选--><divclass="form-group"><selectmultiple="multiple"name=""id=""class="form-control"><optionvalue="">a</option><optionvalue="">b</option><optionvalue="">c</option><optionvalue="">d</option><optionvalue="">e</option></select></div></form><scriptsrc="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script><!--最新的Bootstrap核心JavaScript文件--><scriptsrc="https://cdn.bootcss.com/bootstrap/3.3.7/js/bootstrap.min.js"integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa"crossorigin="anonymous"></script></body></html>