According to paragraph 3, why is the height of a raised beach not a straightforward indicator of past sea level? A. The height of the beach may have been raised or altered by human interventions in the shore environment. B. The height of the beach may be the result of erosion over a brief period of time. C. It is difficult to assess if the sea actually existed at that level for any significant period of time. D. The surface of Earth may have shifted, moving the beach from its original positron.