文勇上的一道题，大家帮我看看吧~

lijugong

Paragraph 4: 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．
8.According to paragraph 4，it was difficult for Smith to distinguish rock strata because
A.the rocks from different strata closely resembled each other
B.he was often unable to find fossils in the younger rock strata
C.their similarity to each other made it difficult for him to distinguish one rock type from
another
D.the type of rock between two consistent strata was always the same
请问C和D有什么区别啊，我觉得都差不多啊！！！