CodingForums.com - jQuery : how to parse selected value to another text box

here is my iQuery code :

Code:

<html>

<head>

<title>Selects jQuery parsed..</title>

<script type="text/javascript" src="jquery-1.8.3.min.js"></script>

<script type="text/javascript">

$(document).ready(function(e) {



$("#no_exam").change(function(){

var textval = $(":selected",this).val();

$('input[name=std_marks]').val(textval);

result_form.std_marks.disabled=(!textval)?false:true; // if select a value , text feild is readonly



})



});</script>

</head>



<body>

Selection :<select name="no_exam" id="no_exam">

<option value="" selected="selected">===select option===</option>

<option value="A+">100-75</option>

<option value="B+">50-74</option>

<option value="C+">0-49</option>

<option value="AB">AB</option>

<option value="NE">NE</option>

<option value="MC">MC</option>

</select></br>

Selected value :<input type="text" name="std_marks" id="std_marks"/>

</body>

</html>

this code is works perfectly for me...But I want to change this code ..

my problem is here this selection group have two categories...one is marks & other is absent..

* Marks category:100-75,50-74,0-49

* Absent category :AB,NE,MC

Here when I select a option then it's value is parse to the Selected value box..How can I edit this code to parse values only select Absent category options....(When I select a Marks category option ,do not parse a its value )