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求大神，為什么運(yùn)行不了

!DOCTYPE html>
<html>
?<head>
? <title> 事件</title>?
? <script type="text/javascript">
?? function count(){
?????? var m = parseInt(document.getElementById("txt1").value);
?????? var n = parseInt(document.getElementById("txt2").value);
?????? var z=document.getElementById("select").value;
???????
?????? var num;
?????? switch(z){
?????????? case'+';
?????????? num=m+n;
?????????? break;
?????????? case'-';
?????????? num=m-n;
?????????? break;
?????????? case'*';
?????????? num=m*n;
?????????? break;
?????????? case'/';
?????????? num=m/n;
?????????? break;
?????? }
?????? alert(num);
?????? document.getElementById("fruit").value=num;
?? }
? </script>
?</head>
?<body>
?? <input type='text' id='txt1' />
?? <select id='select'>
??<option value='+'>+</option>
??<option value="-">-</option>
??<option value="*">*</option>
??<option value="/">/</option>
?? </select>
?? <input type='text' id='txt2' />
?? <input type='button' value=' = ' onclick="count()"> 
?? <input type='text' id='fruit' />??
?</body>
</html>