What Is the Likely Connection Between the Metal Iridium and the Demise

What is the likely connection between the metal iridium and the demise of Earth's dinosaur population?

A) Iridium, which is found in abundance on Earth's surface, is poisonous to reptiles.
B) Iridium is found beneath Earth's crust. Meteor impacts during the dinosaur age probably exposed and uncovered enough of it to poison the dinosaurs.
C) Iridium is found in meteorites but is rare on Earth. The existence of a worldwide layer of it suggests a large meteor impact during the dinosaur age. The impact probably raised enough dust to block out sunlight and kill the dinosaurs.
D) Iridium is highly radioactive. Its presence in a geologic layer dating to the dinosaur age suggests that natural radioactivity reached dangerous levels at that time, and the dinosaurs died from overexposure.