Dinosaurs were reptiles which became extinct about 65 million years ago. The most intrigui

ng question about dinosaurs has always been "【21】did they die out？" There is no simple answer to this question,【22】many hundreds of scientists are studying the problem. They are not studying the extinction of the dinosaurs【23】, but the whole question of extinction. Many other plants and animals have【24】in the past, and it is important to understand【25】this happened. Having this information could help save many species that are【26】in the modern world. Humans are causing extinctions now, because of pollution and other damage【27】the environment. Maybe the dinosaurs can tell us how to save the earth today,【28】their extinction 65 million years ago! Some of the early dinosaur scientists, 100 years ago, thought the dinosaurs died out because the【29】changed, and they could not breathe. Others thought that the dinosaurs disappeared simply because they became too big.

They were【30】to move and could not find enough food【31】One theory is that a huge killer meteorite【32】the earth. Some scientists【33】that the extinction of dinosaurs was possibly due to rapid【34】of the planet's climate. Perhaps huge amounts of lava pours out of volcanoes in India. This sent up vast【35】of dust that blacked out the sun, and made the earth icy cold.

(51)

A．when

B．why

C．how

D．/