What’s the primary reason that mammals an avian dinosaurs thrived in the paleogene period

B. Their predators have been killed by the Cretaceous-Paleogene extinction

Explanation:

The mammals and the avian dinosaurs were small creatures, living in the shadows of the dinosaurs which dominated the world, but also were stopping the further increase in size and development of these animals. At the end of the Cretaceous though, an asteroid struck the Earth. The impact was so hard and devastating that it managed to destroy the dinosaurs and wipe them out of the face of the Earth, with the only dinosaurs remaining being the small avian species. Since lots of niches opened up and where free for taking in the ecosystem, the mammals and the avian dinosaurs quickly started to evolve, diversify, and become much larger. The mammals seem to won the race for dominance as they have become the most dominant species on the planet.