The best index fossils tend to be marine creature. There animals evolved rapidly and spread over large over large areas of the world.
The earliest animals whose remains have been found were all very simple kinds and lived in the sea. Later forma are more complex,and among these are the sea?lilies, relations of the star?fishes, which had long arms and were attached by a long stalk to the sea bed, or to rocks.
When an animal dies,the body, its bones, or shell, may often be carried away by streams into lakes or the sea and there get covered up by mud. If the animal lived in the sea its body would probably sink and be covered with mud. More and more mud would fall upon it until the bones or shell become embedded and preserved.
Many factors can influence how fossils are preserved in rocks. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form.
[试题分析]
本篇是一篇科普文章,介绍的是史前类动物。文章结构比较清晰,首先用一般性的介绍来引入史前类动物和记载它们形体和活动的化石,接下来的几段就逐渐按照进化的顺序来一一介绍各种动物。
具体分析一下五个空白处的内容。
41.首先来看一下空白处的上下文。上文讲的是许多生物都已经灭绝,没有后代生活在这个世界上。下文讲的是化石使我们了解了这些生物的形态。可以推断,空白处是一个衔接的句子,应该是从生物到化石介绍的过渡,再看选项中,[A][B][E]都提到了这两方面内容,但[A][E]谈的是具体的一些生物,不是这段泛泛而谈的所有生物,因此不契合。[B]是正确答案。
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