option.append() doesn't work

Wonderfull plugin, but I have problems to create two select input depending each other (the second select should be populated when the user select a value in the first select input).
My ajax function return (correctly) a list of option (for example: "AAAABBBBCCCC"
I use the append method on success
success: function (data) {
// alert( data ); <-- this alert works fine and show me the option list
$('#cerca_dist').append(data);
}

#cerca_dist is the select input I'm trying to populate with ajax response

where I'm wrong?