In paragraph 2, the author discusses the presence of iridium in the sediment layer discovered by the Alvarezes because A. it supported their claim that toxic levels of iridium could have killed the dinosaurs B. it led some scientists to question the validity of the Alvarezes' study C. it led the Alvarezes to conclude that a space impactor killed the dinosaurs D. it showed that new discoveries were still being made about the extinction of the dinosaurs