如何获得基于所选州的城市列表？

Question

为什么我的AJAX调用未将数据传递到给定的URL？它也没有显示任何错误。如何获得基于所选州的城市列表？

<label>State</label>
<select name="state" id="state">
  <option value="" disabled selected>Choose your state</option>
  <?php foreach($states as $state){
    echo '<option value="'.$state['state_id'].'">'.$state['state_name'].'</option>';
  } ?>
</select><br><br>

<label>Aperture Place</label>
<select name="aperture_place" class="city">
  <option value="" disabled selected>Choose your city</option>
</select><br><br>

$(document).ready(function() {
  jQuery('#state').change(function() {
    var id = jQuery(this).val();
    //alert(id);
    jQuery.ajax({
      type: 'post',
      url: "<?php echo base_url().'routes/get_city' ?>",
      data: 'id=' + id,
      success: function(result) {
        alert(result);
        //jQuery('.city').html(result);
      }
    });
  });
});

public function get_city() 
{
  $id = $_POST['id'];
  $query = $this -> common -> select_data_by_id('city', 'state', $id, '*', '');
  print_r($query);
  exit();
  echo '<option value="" disabled selected>Choose your city</option>';
  foreach($query as $place) {
    echo '<option value="'.$place['city_name'].'">'.$place['city_name'].'</option>';
  }
}