TOEFL Reading: ETS-TOEFL阅读机经 - PRX1X6HI8718O36QL$

According to paragraph 6, what is one reason that complex geological analyses are needed for locating many of the earliest Nile River communities A. Flood waters can carry evidence of human habitation far from its place of origin. B. The streams associated with early communities have changed course over time. C. More recent communities have been built on top of ancient communities. D. The types of sediments carried by Nile floods have changed since ancient times.