syntax for retaining value in a drop down box

Please kindly tell me how to retain a value in the drop down box. I mean if i have a form and i select one option from the drop down box, and when i submit the form, if something is wrong in the submission, the very form is returned with all the values intact. But incase of me i am not able to retain the value in the drop down box. Please see my code:

[PHP]
<td class="smallfie ldcell">
<?php echo $form->error("class") ; ?>
<select name="class" tabindex="2" id="class"><?ph p echo $form->value("class") ; ?>
<option>Selec t</option>
<option value="A">A</option>
<option value="B">B</option>
<option value="C">C</option>
<option value="D">D</option>
</select>
</td>
[/PHP]

requesting for your urgent reply.