Drupal 7 dependent dropdown not working in form alter - drupal-7

Question

I am trying to implement the dependent dropdown option in Drupal 7 from this URL http://w3shaman.com/article/creating-ajax-dropdown-drupal-7. When changing the first dropdown value, it is showing the loading indicator but no value is fetching in dependent dropdown
.
. Here is the code I have tried so far.
This is the code of form alter module
function xyz_mod_alter(&$form, &$form_state, $form_id) {
$form['sandbox_ajax_dropdown']['province'] = array(
'#title' => t('Province'),
'#type' => 'select',
'#options' => _load_province(),
'#ajax' => array(
'event'=>'change',
'callback' =>'sandbox_ajax_dropdown_city',
'wrapper' => 'city-wrapper',
),
);
// Wrapper for city dropdown list
$form['sandbox_ajax_dropdown']['wrapper'] = array(
'#prefix' => '<div id="city-wrapper">',
'#suffix' => '</div>',
);
// Options for city dropdown list
$options = array('- Select city -');
if (isset($form_state['values']['province'])) {
// Pre-populate options for city dropdown list if province id is set
$options = _load_city($form_state['values']['province']);
}
// Province dropdown list
$form['sandbox_ajax_dropdown']['wrapper']['city'] = array(
'#title' => t('City'),
'#type' => 'select',
'#options' => $options,
);
}
Code for loading the default value of province and fetching city data function
function sandbox_ajax_dropdown_city($form, $form_state) {
// Return the dropdown list including the wrapper
return $form['sandbox_ajax_dropdown']['wrapper'];
}
function _load_province() {
$province = array('- Select province -');
$query = db_query("select tdd.tid as term_id, tdd.name as name
from taxonomy_term_data td
join taxonomy_vocabulary v on v.vid=td.vid
join taxonomy_term_hierarchy th on td.tid=th.parent
join taxonomy_term_data tdd on th.tid=tdd.tid
where td.tid='702269'
and v.name ='xx' and tdd.weight!=-1 ");
foreach($query as $result){
$province[$result->term_id] = $result->name;
}
return $province;
}
function _load_city($province_id) {
$city = array('--- Select city ---');
$query = db_query("select tdd.tid as term_id, tdd.name as name
from taxonomy_term_data td
join taxonomy_vocabulary v on v.vid=td.vid
join taxonomy_term_hierarchy th on td.tid=th.parent
join taxonomy_term_data tdd on th.tid=tdd.tid
where td.tid=".$province_id."
and v.name ='xx' and tdd.weight!=-1 ");
foreach($query as $result){
$city[$result->term_id] = $result->name;
}
return $city;
}
I am not sure about the mistake I am doing. I copied the exact code from the given link. Even I tried to change as per my need but it is not working.