TOEFL Listening: TPO-TOEFL阅读TPO - G458S96054QBZ35ZN$

According to paragraph 6, what made iridium a useful test of the Alvarez hypothesis? A. Its occurrence in a few locations on Earth against several locations on other planets B. Its occurrence in limited quantities on Earth against its abundance in asteroids C. Its ability to remain solid at extremely high temperatures D. Its ease of detection even in very small amounts