How to use a protected function from another class

Class A

PHP Code:

<?php
namespace App\Controllers;
use CodeIgniter\Controller;
class A extends Controller
{
    protected function a()
    {
        return 'a';

    }
} 


Class B：

PHP Code:

<?php
namespace App\Controllers;
use CodeIgniter\Controller;
use App\Controllers\A;
class Test extends Controller
{
    private $a;
    function __construct()
    {
        $this->a= new A();
    }
    function b(){
        $b = $this->a->a();
        return $a;
    }
} 


Error
Call to protected method...

How can I get the 'a' in Class B