Control Show Hide Form elements depending on selected value

form.html

<form accept-charset="UTF-8" action="#" class="search_aside" id="advanced_search" method="post">
  <div class="row">
    <select id="practice_area" name="advanced_search[law_area_id]">
      <option value="">Select Area of Practice</option>
      <option value="14">Admiralty &amp; Maritime Law</option>
      <option value="24">Banking and Finance Law</option>
    </select>
  </div>
  <div class="row">
    <select id="advanced_search_country_id" name="advanced_search[country_id]"><option value="">Select Country</option>
      <option value="1">Kenya</option>
      <option value="2">Tanzania</option>
    </select>
  </div>
  <div class="row">
    <select id="advanced_search_county_id" name="advanced_search[county_id]">
      <option value="">Select County</option>
      <optgroup label="Kenya">
        <option value="1">Nairobi</option>
        <option value="2">Mombasa</option>
      </optgroup>
      <optgroup label="Tanzania">
        <option value="53">Dar es salaam</option>
        <option value="54">Zanzibar</option>
      </optgroup>
      <optgroup label="Rwanda">
        <option value="63">Kigali</option>
        <option value="64">Butare</option>
      </optgroup>
      <optgroup label="Uganda">
        <option value="73">Kampala</option>
        <option value="74">Gulu</option>
      </optgroup>
      <optgroup label="Burundi">
        <option value="83">Bujumbura</option>
        <option value="84">Muyinga</option>
      </optgroup>
    </select>
  </div>
  <div class="row">
    <input class="button" id="button" type="submit" value="Search">
  </div>
</form>

script.js

jQuery(function() {
  var counties;
  $('#law_firm_county_id').parent().hide();
  counties = $('#law_firm_county_id').html();
  return $('#law_firm_country_id').change(function() {
    var country, options;
    country = $('#law_firm_country_id :selected').text();
    options = $(counties).filter("optgroup[label='" + country + "']").html();
    if (options) {
      $('#law_firm_county_id').html(options);
      return $('#law_firm_county_id').parent().show();
    } else {
      $('#law_firm_county_id').empty();
      return $('#law_firm_county_id').parent().hide();
    }
  });
});