Coastal lagoons in southern Iberia contain sediment layers from around 4,000 years ago in which brackish‐tolerant diatoms become markedly more abundant. On its face, this shift suggests a drier climate, since reduced freshwater inflow from rainfall would have caused salts to concentrate. But the same pattern could have arisen if early farming communities diverted streams to irrigate fields. Here the scale of the observed change becomes decisive. Large, region‐wide hydrological alterations would require the kind of extensive canal and reservoir systems that did not emerge until many centuries later; human impacts around 4,000 years ago would therefore have been local, not regional. The diatom shift, documented across multiple basins far from known early settlements, is thus more plausibly read as a climatic signal.

1. The highlighted sentence has which of the following functions in the passage?

A. It provides a concrete example of a difficulty introduced earlier.
B. It questions the reliability of the dates assigned to early irrigation systems.
C. It describes the nature of local water‐management practices at that time.
D. It offers evidence in support of a new hypothesis.
E. It restricts the spatial scale at which a competing explanation can plausibly operate.


2. The passage indicates which of the following about the diatom shift observed around 4,000 years ago?

A. Human diversion of freshwater was the primary cause of the shift.
B. The shift most likely signaled a move toward drier conditions.
C. Large‐scale irrigation systems were already widespread by that time.
D. The shift occurred only near known early farming settlements.
E. Climatic change and human activity left identical sedimentary signals at that time.