Add fields with multiple options Prestashop 1.6

[cyberespia]

I added 3 new fields to my prestashop 1.6 according to the following steps_:

 

1. 

ALTER TABLE `ps_product` ADD `c_duration` varchar(255);
ALTER TABLE `ps_product` ADD `c_year` varchar(255);
ALTER TABLE `ps_product` ADD `c_studios` varchar(255);

2. Upload to override/classes/--> Product.php

<?php
Product::$definition['fields']['c_duration'] = array('type' => ObjectModel::TYPE_INT, 'validate' => 'isInt');
Product::$definition['fields']['c_year'] = array('type' => ObjectModel::TYPE_STRING, 'validate' => 'isString');
Product::$definition['fields']['c_studios'] = array('type' => ObjectModel::TYPE_STRING, 'validate' => 'isGenericName', 'values' => array('option a', 'option b'), 'default' => 'option a');



class Product extends ProductCore
{	
	public $c_duration = 0;	
	public $c_year = '';
	public $c_studios = '';
}

3.En admin theme add:  admin/themes/default/template/controllers/products/informations.tpl:

	<div class="form-group">
		<label class="control-label col-lg-3" for="c_duration">
			<span class="label-tooltip" data-toggle="tooltip"
			title="{l s='Duration'}">
				{l s='Duration'}
			</span>
		</label>
		<div class="col-lg-5">
			<input type="text" id="c_duration" name="c_duration" value="{$product->c_duration|htmlentitiesUTF8}" />
		</div>
	</div>

	<div class="form-group">
		<label class="control-label col-lg-3" for="c_duration">
			<span class="label-tooltip" data-toggle="tooltip"
			title="{l s='Year Film'}">
				{l s='Year Film'}
			</span>
		</label>
		<div class="col-lg-5">
			<input type="text" id="c_year" name="c_year" value="{$product->c_year|htmlentitiesUTF8}" />
		</div>
	</div>



	<div class="form-group">
		<label class="control-label col-lg-3" for="c_duration">
			<span class="label-tooltip" data-toggle="tooltip"
			title="{l s='Studios Film'}">
				{l s='Studios Film'}
			</span>
		</label>
		<div class="col-lg-5">

			<select name="c_studios" id="c_studios">				
                        <option value="option a" {if $product->c_studios == 'option a'}selected="selected"{/if}> option a </option>
                        <option value="option b" {if $product->c_studios == 'option b'}selected="selected"{/if}> option b </option>
			</select>
		</div>
	</div>

3. and add in my  template  /public_html/alquilerdepeliculas/themes/default-bootstrap/product.tpl

			<p>Duration: {$product->c_duration}</p>
			<p>Year: {$product->c_year}</p>
			<p>Studio: {$product->c_studios}</p>			

All is working correctly.

 

 

My question is how do I make a  field with "select multiple" (in my previous example for the field: "c_studios")

 

 

In HTML is: 

<select multiple="multiple">
        <option value="Opcion 1">Opcion 1</option>
        <option value="Opcion 2">Opcion 2</option>
</select>

I would appreciate your help.