写一个函数，告知你选的小动物的叫声
<html>
<head>
</head>
<body >
<script type="text/javascript">
function scream(animal) {
switch(animal){
case "dog":
document.write('wangwang');
return;
case "cat":
document.write('miao');
return;
case "sheep":
document.write('mie~mie~');
return;
}
}
</script>
</body>
</html>
