How do you add a class to a form dropdown?

[eluser]Wonder Woman[/eluser]
I have a dropdown with lots of options but was just wondering how I add a class to it?

Code:

$property_type = array(
   '0' => 'Any',
   '1' => 'Flats / Apartments',
   '2' => 'Terraced',
   '3' => 'Semi-detatched',
   '4' => 'Detatched',
   '5' => 'All Houses'
);

How do I put:

Code:

class => 'property'

into:

Code:

form_label('Property Type:', 'property_type').' '.form_dropdown('property_type', $property_type);

Thanks