单选题
Dinosaurs were reptiles which became extinct about 65 million years ago. The most【C1】______question about dinosaurs has always been【C2】______did they die out?" There is no simple answer to this question, 【C3】______many hundreds of scientists are studying the problem. They are not studying the extinction of the dinosaurs【C4】______, but the whole question of【C5】______Many other plants and animals have【C6】______in the past, and it is important to understand【C7】______this happened. Having this information could help save many species that are【C8】______in the modern world. Humans are causing extinctions now, because of pollution and other damages【C9】______the environment. Maybe the dinosaurs can tell us how to save the earth today, 【C10】______their extinction 65 million years ago? Some of the early【C11】______scientists, 100 years ago, thought the dinosaurs died out because the【C12】______changed, and they could not breathe. Other thought that the dinosaurs disappeared simply because they became too big. They were【C13】______to move without falling over, and could not find enough food【C14】______One theory is that a huge killer meteorite【C15】______the earth. Some scientists【C16】______that the extinction of dinosaurs was possibly due to rapid【C17】______of the planet' s climate. Perhaps huge amount of lava pours out of【C18】______in India. This sent up vast【C19】______of dust that blacked out the sun, and made the earth【C20】______cold.