Read the text below and choose the answer A, B, C or D which you think fits best according to the text. The latest development in the debate amongst scientists about what killed the prehistoric dinosaurs is the suggestion that acid rain was the cause. Two geologists from the Massachusetts Institute of Technology suggest that a large meteor hitting the earth at 65 kilometres per second would have led to strongly acidic rain falling all over the world. The two geologists, Ronald Prinn and Bruce Fegley, calculated that if an ice-rich meteor weighing 12.5 million billion kilograms, hit the Earth, it would shock-heat the atmosphere enough to produce huge amounts of nitrogen oxides. This would result, they say, in strongly acid rain around the world. If the meteor were travelling more slowly, or if it were more rock-like in structure, this strongly acid rain would be limited to a small area so that the world effect would be much less important. However, other groups of researchers have suggested that volcanoes, rather than meteors, could have produced these heavy doses of acid rain, but over a much longer period — over 10,000 years. Prinn and Fegley believe that their theory matches the pattern of animal and plant life at the end of the Cretaceous period 65 million years ago. It is well known from studying rock formations that it was at this time that dinosaurs and many other groups of prehistoric animals died out. Prinn and Fegley suggest one way in which their theory might be tested. Rock layers which have remained undisturbed since the Cretaceous era could be examined to see if they show any sign of the effects of very large amounts of acid rain on metals which do not dissolve in water such as iron and lead. Such undisturbed rock layers are, however, very rare. 51: What do scientists know about prehistoric dinosaurs? A. They were killed by one vast volcanic explosion. B. They were present on the earth for about 10,000 years. C. They could not co-exist with other animals. D. They died out 65 million years ago. 52: What do Prinn and Fegley think would happen if a large piece of ice-rich meteor hit the earth at great speed? A. A huge volcano would be formed. B. 12.5 million billion kg of nitrogen oxide would be produced. C. Strongly acidic rain would result all over the world. D. The heat would kill all living things in the area. 53: Prinn and Fegley think that if a rocky meteor hit the earth it would A. have little effect on things living nearby. B. have very long-lasting effects. C. take a long time to have any effect. D. not have any important world-wide effects. 54: Prinn and Fegley suggest that scientists could check their theory by looking in rock layers for A. signs of the effect of rain on metals. B. the preserved bones of dinosaurs and other animals. C. evidence of high water levels in prehistoric times. D. disturbances between the different layers of rock. 55: What problem exists about doing the check that Prinn and Fegley have suggested? A. Few people have enough skill. B. Massachusetts Institute of Technology cannot do any more research. C. Recent acid rain has damaged the rocks. D. Most rock layers have been disturbed since the Cretaceous era.