【正确答案】The professor explains that there are two theories to explain the sudden disappearance of dinosaurs 65 million years ago. One is called the Impact Theory. It's a widely supported theory, and it suggests that dinosaurs died out after an asteroid impact. When a gigantic asteroid collided with Earth, the sky was darkened with dust, and sunlight couldn't get through the dust cloud. As a result, plants died, the entire food chain suffered, and dinosaurs eventually died out. The other is called the Volcano-Greenhouse theory. It explains that there was a big increase in volcanic eruptions. The eruptions threw a lot of carbon dioxide into the atmosphere, and this produced a greenhouse effect. The Earth's temperature warmed, and it became so hot that dinosaurs were no longer able to reproduce, which of course led to their extinction.

【答案解析】[听力原文] Now, class, I'm sure you've read and heard a lot about the extinction of the dinosaurs, which occurred about 65 million years ago. People are fascinated by this topic, probably because we have yet to determine its cause. Why did these giant reptiles, who had ruled the planet for millions of years, suddenly disappear? Well, the theories are endless, but I'd like to talk about two that are the most plausible. The first is known as the Impact Theory, and I'd say it's the most widely supported. According to this concept, there was a very large asteroid-like object that collided with Earth. The results were catastrophic: enormous explosions, earthquakes, tsunamis, raging forest fires...But most importantly, an impact of that magnitude would have thrown up large amounts of dust into the sky, so much that it would've prevented sunlight from reaching the surface. According to the theory, plants would have died first from the lack of light, and then this decline worked its way up the food chain, until the extinction of the dinosaurs was complete. As I said, it's a very well-supported theory, but there's another one, and it's called the Volcano-Greenhouse Theory. This idea holds that there was a significant increase in volcanic eruptions 65 million years ago. These eruptions released large amounts of carbon dioxide into the atmosphere, creating a greenhouse effect and causing temperatures on Earth to rise. Now, this wasn't enough to kill the dinosaurs immediately. What happened, according to the theory, was that the heated environment made it impossible for dinosaur embryos to survive. Female dinosaurs were unable to produce offspring, and, well, you can see how that would lead to the extinction of species.