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                        /JavaScript入門篇

為什么這段getElementsByClassName沒有效果，而換成getElementByID就有效果了

<!DOCTYPE?html>
<html?lang="en">
<head>
????<meta?charset="UTF-8">
????<title>mapDr2</title>
????<style>
?.btns_mapDr{
????????????color:?red;
?border-color:?#66ccff;
?border-style:dotted;
?border-width:3px;
?}
????????.btns_mapDr:hover{
????????????cursor:?pointer;
?}
????</style>
</head>
<body>
<table>
????<tr>
????????<td?class="btns_mapDr?mapDrbili"?id="mapDrbili"??mapURL-data="http://www.bilibili.com/video/kichiku.html">鬼畜區</td>
????????<td?class="btns_mapDr?mapDracfun"?id="mapDracfun"??mapURL-data="http://www.bilibili.com/video/teleplay.html">影視區</td>
????</tr>
</table>
<iframe?id="mapFrame"?width="100%"?height="800"?src?="https://www.bilibili.com/"></iframe>
</body>
<script?type="text/javascript">
?var?btns?=?document.getElementsByClassName("btns_mapDr");
?btns.onclick?=?function?()?{
????????alert("hello");

?}
</script>
</html>