How shall i add class[CSS] in dropdown

[eluser]Sumon[/eluser]
This is the way to create drop down in codeigniter:
$options = array(
'small' => 'Small Shirt',
'med' => 'Medium Shirt',
'large' => 'Large Shirt',
'xlarge' => 'Extra Large Shirt',
);
echo form_dropdown('shirts', $options, 'large');

How shall i add class[CSS] for this which will produce:
<select name="shirts" class="listShift">

For your kind information i need the solution of Class not Style.

Thanks