Question 20 is based on the recording you have just heard.

        The partial skeletons of more than 20 dinosaurs and scattered bones of about 300 more have been discovered in Utah and Colorado at what is now the Dinosaur National Monument. (19) [Many of the best specimens may be seen today at museums of natural history in the larger cities of the United States and Canada.] This dinosaur pit is the largest and best preserved deposit of dinosaurs known today. (20) [Many people get the idea from the massive bones in the pit wall that some disaster, such as a volcanic explosion or a sudden flood, killed a whole herd of dinosaurs in this area. ]This could have happened, but it probably did not.

        The main reasons for thinking otherwise are the scattered bones and the thickness of the deposit. In other deposits where the animals were thought to have died together, the skeletons were usually complete and often all the bones were in their proper places. Rounded pieces of fossil bone have been found here. These fragments got their smooth, round shape by rolling along the stream bottom. In a mass killing, the bones would have been left on the stream or lake bottom together at the same level. But in this deposit, the bones occur throughout a zone of sandstone about 12 feet thick. The mixture of swamp dwellers and dryland types also seems to indicate that the deposit is a mixture from different places.

        The pit area is a large dinosaur graveyard—not a place where they died. (21) [Most of the remains probably floated down an eastward flowing river until they were left on a shallow sandbar. ]Some of them may have come from far-away dryland areas to the west. Perhaps they drowned trying to cross a small stream, or were washed away during floods. Some of the swamp dwellers may have got stuck in the very sandbar that became their grave. Others may have floated for miles before being stranded.

        Even today, similar events take place: when floods come in the spring, sheep, cattle, and deer are often trapped by rising waters and often drown. Their dead bodies float downstream until the flood recedes, and leaves them stranded on a bar or shore where they lie, half buried in the sand, until they decay. Early travelers on the Missouri River reported that shores and bars were often lined with the decaying bodies of buffalo that had died during spring floods.

20. What occurs to many people when they see the massive bones in the pit wall?（当很多人看到坑壁上的巨型骨骼时，他们会想到什么？）

解析：B。录音中间指出，很多人在看到坑壁上的巨型骨骼时会猜想，或许是某场灾难，例如火山爆发或突如其来的洪水，导致了此处一大群恐龙的死亡。B项中的Some natural disaster是对录音中具体的some disaster, such as a volcanic explosion or a sudden flood的概括，后半句的killed a whole heard of dinosaurs in the area属于原词复现，故B项为正确答案。

错项排除：录音只是说火山爆发和洪水是人们猜测导致恐龙死亡的原因，并没有说火山爆发让整个物种灭绝或被洪水侵袭，A、D两项属于断章取义，故排除。录音中没有提到人们觉得应该保护恐龙坑以用作研究，故C项排除。