谁能告诉我这个简易计算器错哪了，onclick的时候没反应呢
<script type="text/javascript">
function count(){
var num="";
var t1= document.getElementById("txt1").value;
var t2= document.getElementById("txt2").value;
var sel=document.getElementById("select").value;
switch(sel){
case "+": num = parseInt(t1) + parseInt(t2); break;
case "-": num = parseInt(t1) - parseInt(t2); break;
case "*": num = parseInt(t1) * parseInt(t2); break;
default : num = parseInt(t1) / parseInt(t2); break;
}
document.setElementById("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()"/> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
