As he collected fossils from strata throughout England, Smith began to see that the fossils told a different story from the rocks. Particularly in the younger strata, the rocks were often so similar that he had trouble distinguishing the strata, but he never had trouble telling the fossils apart. While rock between two consistent strata might in one place be shale and in another sandstone, the fossils in that shale or sandstone were always the same. Some fossils endured through so many millions of years that they appear in many strata, but others occur only in a few strata, and a few species had their births and extinctions within one particular stratum. Fossils are thus identifying markers for particular periods in Earth's history.
According to paragraph 4, it was difficult for Smith to distinguish rock strata because
○the rocks from different strata closely resembled each other
○he was often unable to find fossils in the younger rock strata
○their similarity to each other made it difficult for him to distinguish one rock type from another
○the type of rock between two consistent strata was always the same
定位句是「The rocks were often so similar that he had trouble distinguishing the strata, but he never had trouble telling the fossils apart.」没错，这句话中的so…that…因果关系一定不要读漏掉。考点确定之后，仔细阅读因果话题是指rocks were similar ？？ he had trouble distinguishing the strata.