Final answer:
Hadrosaurs' main defense mechanism was indeed to run, as their physical structure suggests they were built for speed rather than combat. Living in large herds also likely played a role in their survival against predators.
Step-by-step explanation:
The statement that hadrosaurs' main defense mechanism was running is true. Hadrosaurs were a group of herbivorous dinosaurs known as 'duck-billed dinosaurs' for their flattened snouts. While they possessed no significant physical defenses such as sharp claws or teeth for fighting predators, their strong legs and stiff tail suggest they could move quickly to escape from predators, much like modern-day prey animals do. This swiftness would have been their primary means of defense. Additionally, like many modern herbivores, hadrosaurs likely benefited from living in large herds, which could deter predators and provide safety in numbers.
Considering fossil evidence, adaptations such as long legs relative to torso length indicate swift movement was a key survival trait among many dinosaurs. Similar to how the bipedal Mononykus dinosaur may have relied on its speed to evade predators in the open desert plains, hadrosaurs could have used their ability to run as a main defense strategy in their respective habitats.