Final answer:
The mass extinction at the end of the Cretaceous period, which resulted in the demise of the dinosaurs, was primarily caused by an b) asteroid impact. This catastrophic event led to environmental conditions that were inhospitable for dinosaurs, allowing other life forms to thrive afterwards.
Step-by-step explanation:
At the end of the Cretaceous period, the mass extinction event that led to the demise of the dinosaurs is believed to have been caused by an b) asteroid impact.
A significant piece of evidence for this hypothesis was the discovery of a sharp spike in iridium levels in the geological layer marking the boundary between the Cretaceous and Paleogene periods.
Iridium is a material that is rare on Earth's surface but is commonly found in space.
The magnitude of the asteroid impact and its consequences, such as a rapid influx of iridium, and the associated environmental effects—darkened skies, halted photosynthesis, and altered climates—played a crucial role in causing the mass extinction.
The resulting changes gave rise to new opportunities for other organisms in the subsequent Cenozoic era, notably mammals.