Question

the prevalence of what mostly isotopic and radioactive element in a layer of soil from 65 - 66 million years ago indicates that the dinosaurs were killed by an asteroid in the k - t event?
iridium
azimuth
boron
cadmium

Explanation:

The high - concentration of iridium in the 65 - 66 million - year - old soil layer is evidence for the asteroid impact theory of dinosaur extinction. Iridium is rare on Earth's surface but abundant in asteroids. Azimuth is a direction - related term, not an element. Boron and cadmium are not associated with this event in this context.

Answer:

Iridium