Final answer:
A worldwide layer of rock formed at the end of the Cretaceous period is rich in iridium, a substance linked to the asteroid impact hypothesis for the mass extinction at that time. This iridium layer, along with other geological evidence, supports the theory that an asteroid caused the extinction event.
Step-by-step explanation:
The bolide impact at the end of the Cretaceous period is associated with a worldwide layer of rock that is rich in a substance known for its rarity on Earth's surface but common in asteroids. This substance is iridium, which was discovered in heightened concentrations across the globe in sedimentary layers that mark the boundary between the Cretaceous and Paleogene periods.
This discovery, made by Luis and Walter Alvarez, Frank Asaro, and Helen Michels in 1980, along with the identification of a crater in the Yucatan peninsula, supports the asteroid impact hypothesis for the mass extinction event.
The impact in question struck off the coast of the Yucatán Peninsula, leading to the deposition of iridium-enriched sediment from the resulting dust cloud. The iridium spike, along with the extinction of numerous species including the dinosaurs, and the geological evidence of a large crater, are all part of the evidence supporting the asteroid impact theory of the Cretaceous-Paleogene extinction event. Thus, the correct answer to the question is B. iridium.