在2014年5月的托福阅读考试中有这样一道题：行星对恐龙灭绝有没有影响。针对这道托福考试，留学群(m.liuxuequn.com)小编为各位考生整理了托福阅读背景知识，帮大家普及一下相关的背景知识，这样有助于大家在面对这类题目时方便作答。
托福阅读真题再现：
小行星对恐龙灭绝的影响。一个科学家发现土层中里有很多Ir元素，而Ir元素在地球上少见，因此推断是小行星导致了恐龙灭绝。后面又说了小行星使得气温降低，空气化学组成改变等等也导致恐龙的灭绝，但是一些小的啮齿类动物则存活了下来。
托福阅读相关背景：
参考文章：Meteorite Impact and Dinosaur Extinction
Extinction of the Dinosaurs
Mass Extinctions
背景知识：
Impact event
Biospheric effects
The effect of impact events on the biosphere has been the subject of scientific debate. Several theories of impact related mass extinction have been developed. In the past 500 million years there have been five generally accepted, major mass extinctions that on average extinguished half of all species. One of the largest mass extinction to have affected life on Earth was in the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90% of all species; life on Earth took 30 million years to recover. The cause of the Permian-Triassic extinction is still matter of debate with the age and origin of proposed impact craters, i.e. the Bedout High structure, hypothesized to be associated with it are still controversial. The last such mass extinction led to the demise of the dinosaurs and coincided with a large meteorite impact; this is the Cretaceous–Paleogene extinction event (also known as the K–T or K–P...