VOA慢速英语:巨大的太空岩石撞击地球的频率是以前的两倍

twice – adv. doubled in amount or degree

journal – n. a magazine that reports on things of special interest to a particular group of people

probabilities – n. the chances that something will happen

belt – n. a region that has a lot of a particular thing

extinction(s) – n. the state or situation that results when something, such as a plant or animal species, has died out completely

arbitrary – adj. not planned or chosen for a particular reason

fragile – adj. easily broken or damaged