Why are lean systems also known as pull systems? A. In lean systems, nothing is provided until a downstream demand occurs. That is why a lean system is known as a pull system. B. In robust systems, nothing is provided until a downstream demand occurs. That is why a lean system is known as a punch system. C. In lean systems, nothing is provided until a downstream demand ends. That is why a lean system is known as a pull system. D. In abundant systems, nothing is provided until a downstream demand occurs. That is why a lean system is known as a push system