Final answer:
The two main hypotheses for the Cretaceous-Tertiary mass extinction are the impact of a large asteroid or comet, supported by geological evidence including an iridium spike and an impact crater, and the volcanic activity and climate change, which points to massive volcanic eruptions causing environmental alterations leading to mass extinction.
Step-by-step explanation:
The most probable hypotheses for the Cretaceous-Tertiary mass extinction are: A. The impact event hypothesis suggests a large asteroid or comet struck Earth, causing the mass extinction. B. The volcanic activity and climate change hypothesis posits that significant volcanic eruptions could have led to climate changes resulting in the mass extinction.
The impact event hypothesis is supported by geological evidence, such as a spike in iridium levels found in the rock stratum that marks the Cretaceous-Paleogene boundary, where an abrupt decline in dinosaur fossils occurs. Confirmation of a large impact crater near the Yucatán Peninsula also supports this theory. On the other hand, the volcanic activity hypothesis suggests that massive lava flows, such as the Deccan Traps eruptions, released vast amounts of sulphur dioxides and carbon dioxide, causing climate change and environmental stress which could induce a mass extinction. These hypotheses help explain the sudden disappearance of dinosaurs and many other species around 65 million years ago.