计算器无法计算结果，哪里出错了？
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a=document.getElementById("txt1").value;
var b=document.getElementById("ttxt2").value;
var s=document.getElementById("select").value;
var h="";
switch(s)
{
case "+":
he=parseInt(a)+parseInt(b);
break;
case "-":
he=parseInt(a)-parseInt(b);
break;
case "*":
he=parseInt(a)*parseInt(b);
break;
default;
he=parseInt(a)/parseInt(b);
}
document.getElementById("fruit").value=h;
}
</script>
</head>
<body>
<input id="txt1" type="text" />
<select id="select">
<option type="+">+</option>
<option type="-">-</option>
<option type="*">*</option>
<option type="/">/</option>
</select>
<input id="txt2" type="text" />
<input type="button" value="=" onClick="count()"/>
<input id="fruit" type="text" />
</body>
</html>
