Mass extinctions have occurred repeatedly during the course of Earth's history. One of these events occurred at the end of the Cretaceous Period, during which the dinosaurs became extinct. Mammals flourished after the dinosaurs were gone. This led to a variety of genetically diverse species. Why did the mammals suddenly start diverging into different species after the dinosaurs were gone?

Most of the predators were gone and suddenly niches were available so almost all the offspring were likely to survive and reproduce.
Once the dinosaurs were gone, the mammals started having larger numbers of offspring in each litter to fill the empty niches.
The mammals chose to move into the vacant niches the dinosaurs had occupied and change their body shapes to suit these new environments.
Mammals began mutating rapidly once the dinosaurs died out and were able to create new species based on the mutations.