Question 21 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.

21. What does the speaker suggest about the large number of dinosaur bones found in the pit?（对于在恐龙坑中发现的大量恐龙骨骼，讲话者说了什么？）

解析：A。录音后半部分提到恐龙的大部分遗骸可能曾沿着一条向东的河漂流而下，最后被留在了一处浅滩沙洲上。其中A项的floated down an eastward flowing river属于原词复现，故为正确答案。

错项排除：录音中没有提及恐龙被埋藏了多长时间，故B项排除。录音中提到既有swamp dwellers又有dryland types，并明确说到这些恐龙化石可能来自不同地区，C项与此内容相悖，故排除。录音之前提及了volcanic explosion，但只是人们看到恐龙骨骼时的猜测，并不是讲话者的观点，故D项错误。