能帮我看下哪里错了吗，点击按钮不显示结果
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a=document.getElementById(txt1).value;
var b=document.getElementById(txt2).value;
var op=document.getElementById(select).value;
var result;
switch(op){
case "+":
result=parseInt(a)+parseInt(b);
break;
case "-":
result=parseInt(a)-parseInt(b);
break;
case "*":
result=parseInt(a)*parseInt(b);
break;
default:
result=parseInt(a)/parseInt(b);
}
document.getElementById(fruit).value=result;
}
</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>
</html>
