Final answer:
The correct answer is option a. The adaptive radiation of mammals was greatly influenced by the mass extinction of dinosaurs, providing new ecological opportunities. Important evolutionary adaptations in mammals, like maintaining constant body temperature and the rise of flowering plants, also played roles in their diversification.
Step-by-step explanation:
The adaptive radiation of mammals is a period of rapid diversification linked with the exploitation of new ecological niches. One major factor promoting this event was thought to be the mass extinction of most dinosaurs, an occurrence that created many opportunities for mammals to fill the ecological roles previously dominated by these reptiles. This mass extinction occurred at the end of the Cretaceous period, around 65 million years ago, primarily due to a catastrophic asteroid impact and massive volcanic eruptions that resulted in dramatic climate changes.
Additionally, during the transition from reptiles to mammals, there were several important physiological changes. Among these were the development of internal fertilization and mechanisms in mammals that allowed for a more constant body temperature.
These adaptations may have benefited mammals over the ectothermic dinosaurs during a drop in Earth's temperature that occurred long before the dinosaurs went extinct. Mammals also used the spread of flowering plants that appeared during this time as a new source of food, either directly or indirectly by consuming insects that fed on these plants.
Furthermore, the diversification of mammals continued with another significant evolutionary event around 50 million years ago when Earth's temperature rose, leading to a greater diversity of plant life. This second event also allowed mammals to develop further, as the increase in temperature and plant diversity provided a bounty of new food sources.