Copy selected option value to another text box but just for some options
<p>HTML excerpt    </p>

<pre><code>Selection:
&lt;select name="no_exam" id="no_exam"&gt;
    &lt;option value="" selected="selected"&gt;===select option===&lt;/option&gt;
    &lt;option value="A+"&gt;100-75&lt;/option&gt;
    &lt;option value="B+"&gt;50-74&lt;/option&gt;
    &lt;option value="C+"&gt;0-49&lt;/option&gt;
    &lt;option value="AB"&gt;AB&lt;/option&gt;
    &lt;option value="NE"&gt;NE&lt;/option&gt;
    &lt;option value="MC"&gt;MC&lt;/option&gt;
&lt;/select&gt;
&lt;/br&gt;

Selected value:
&lt;input type="text" name="std_marks" id="std_marks"/&gt;
</code></pre>

<p>Javascript that is relevant to upper HTML</p>

<pre><code>$("#no_exam").change(function() {
    var textval = $(":selected",this).val();
    $('input[name=std_marks]').val(textval);

    // if value is selected, text field is readonly
    result_form.std_marks.disabled=(!textval) ? false : true;
});
</code></pre>

<p>This code works ok... But I want to change it. My problem is that selection group has two categories... one is marks &amp; other is absent..</p>

<pre><code>--------------------------------------------------
* Marks category: 100-75,50-74,0-49
-----------------------------------------------------
* Absent category: AB,NE,MC
------------------------------------------------------
</code></pre>

<p>Here when I select an option its value gets copied to the input box.</p>

<p>How can I change this code to copy selected option's value when user selects one of the Absent category options but don't do anything when they select a Marks option?</p>

<p>Demo Link:- <a href="http://jsfiddle.net/dontspeak/2zfdq/4" rel="nofollow">Demo Link</a></p>
