为什么点击click3不能弹出alert？？？？
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<head>
<body>
<div id="box">
<input type="button" value="click" id="btn1" onclick="show()">
<input type="button" value="click2" id="btn2">
<input type="button" value="click3" id="btn3">
</div>
<script>
function show(){
alert("hello");
}
var btn2=document.getElementById('btn2');
var btn3=document.getElementById('btn3');
btn2.onclick=function(){
alert("DMO0");
}
btn2.onclick=null;
btn3.addEventListener('click',show, false);
</script>
</body>
</head>
</html>
