Why is there a historical debate surrounding the Great Wall of China? A. The Great Wall was successful at keeping invaders out, but it was a costly project to build. B. The emperor's advisors refused to build the Great Wall although it proved to be a success. C. The cruelty of the emperor caused suffering although the Great Wall was an achievement in defense. D. The Great Wall today is considered an eyesore, although it was necessary in the emperor's time.