Rock layers right at the K-Pg boundary have enriched concentrations of the element iridium.This is significant for understanding the mass extinction that happened at the time,because ________.

A) iridium is toxic to life and could have killed off many species
B) iridium is very rare in the Earth's crust but relatively common in meteorites
C) iridium forms under conditions of extreme shock and heat
D) iridium would have caused an episode of severe acid rain
E) iridium-rich dust is particularly effective at blocking incoming solar radiation