Dirt Bike VS Zombies & Alien

ALERT!!SPECIAL Hidden Reward -- Can you find it? (Hint: Unlock all cars at ONCE.)Alien had invaded Earth.....A lot of our comrades had been turned into Zombies...Dirt Bikers gather all your allies and race through the armies of Aliens and Zombies.Grab as many coins and treasures as you can while you destroy the Aliens and Zombies. However beware!!!! Zombies may catch you from behind.