Reflection error on my first module development I'm using Drupal 8.0.1. Just I'm started to develop a custom module to simply show hello world text in /first path. Here is my code.

FirstController.php [It is in the path sites/all/modules/first_module/src/Controller]

namespace Drupal\first_module\Controller;

use Drupal\Core\Controller\ControllerBase;

class FirstController extends ControllerBase {
  public function content() {
  return array(
      '#type' =&gt; 'markup',
      '#markup' =&gt; t('Hello world'),
    );
  }
}


first_module.routing.yml [It is in the path sites/all/modules/first_module]

first_module.content:
  path: '/first'
  defaults:
    _controller: 'Drupal\first_module\Controller\FirstController::content'
    _title: 'Hello world'
  requirements:
    _permission: 'access content'
    _permission: 'view first'


first_module.info.yml [It is in the path sites/all/modules/first_module]

name: First Module
description: An experimental module to build our first Drupal 8 module
package: Custom
type: module
version: 1.0
core: 8.x


I can able to see my custom module in http://localhost/admin/modules also I can able to install the module. But when I got to the path http://localhost/first it shows the error 

ReflectionException: Class Drupal\\first_module\\Controller\\FirstController does not exist in 
/home/aaa/Public/drupal-8.0.1/core/lib/Drupal/Core/Entity/EntityResolverManager.php on line 128, referer: http://d8.localhost/admin/modules


Where I would went wrong?? Any help!!!
a) in the route definition the path to the controller should start with backslash
b) in the route only one permission will work
c) you do not need to define type markup, just use #markup alone