编写一个html文件的网页代码，页面包含一个下拉列表框、一个文本框和一个按钮，下拉列表框选择要去的网站，当选择完毕后文本框中出现对应的网址。点击确认跳转按钮后访问文本框中出现的网址
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>跳转网址</title>
<script type="text/javascript">
function TxtToChange() {
var url = document.getElementById("_url").value;
switch (url) {
case "https://www.baidu.com/":
document.getElementById("txt").value = url;
break;
case "http://www.sosoapi.com/":
document.getElementById("txt").value = url;
break;
case "https://www.csdn.net/":
document.getElementById("txt").value = url;
break;
}
}
function GoToUrl(){
location = document.getElementById("txt").value;
}
</script>
</head>
<body>
<form>
<select id="_url" onchange="TxtToChange()">
<option selected="selected">请选择</option>
<option value="https://www.baidu.com/">百度</option>
<option value="http://www.sosoapi.com/">sosoApi</option>
<option value="https://www.csdn.net/">CSDN</option>
</select>
<input type="text" id="txt" />
<input type="button" value="确认跳转" onclick="GoToUrl()">
</form>
</body>
</html>