运行不出来，求高手指点
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a=document.getElementById("txt1").value;
var b=document.getElementById("select").value ;
var c=document.getElementById("txt2").value ;
switch(b)
{
case "+":
d= parseInt(a)+parseInt(c);
break;
case "-":
d= parseInt(a)-parseInt(c);
break;
case "*":
d= parseInt(a)*parseInt(c);
break;
case "/":
d= parseInt(a)/parseInt(c);
break;
}
var d=document.getElementById("fruit").value
}
</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>
