What is one reason the presence of Iridium leads scientists to hypothesize that a meteorite impact caused the mass extinction of dinosaurs?

A) There are no fossils of dinosaurs in rocks below the Iridium layer.
B) Iridium is only common on Earth and is never found in meteorites.
C) All dinosaur fossils contain iridium.
D) There are no fossils of dinosaurs in rocks above the Iridium layer.