A、In the deserted fields. B、In the dinosaur pit in Utah. C、In the biology department of big universities. D、At museums of natura

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. 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. 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 the 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 dry-land 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. 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 dry-land 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 the 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 often were lined with the decaying bodies of buffalo that had died during spring floods.
   Questions 19 to 21 are based on the recording you have just heard.
   19.Where can many of the best dinosaur specimens be found in North America?
   20.What occurs to many people when they see the massive bones in the pit wall?
   21.What does the speaker suggest about the large number of dinosaur bones found in the pit?