What's the primary reason that mammals and avian dinosaurs thrived in the paleogene period? a. plants developed as the main food source for these heterotrophic organisms. b. their predators had been killed off by the permian extinction. c. their predators had been killed off by the cretaceous-paleogene extinction. d. the earth significantly warmed, and cold-blooded animals couldn't survive.

The correct answer is option B, that is, their predators had been killed off by the Permian extinction.

Explanation:

The avian dinosaurs and the mammals were small creatures, living under the pump of the dinosaurs that reign the world, however, also were ceasing the further enhancement in the development and the size of these species. At the termination of the Cretaceous era, due to the colliding of an asteroid on the Earth, all the dinosaurs got wiped out from the Earth's surface, with the only remaining one of the species of dinosaurs were the small avian species.  

Due to the opening of lots of niches, the avian dinosaurs and the mammals briskly began to diversify, evolve, and become much bigger in size. The mammals won the race for supremacy and they became the most prevailing species on the Earth.