All of the Following Statements Are Clues to Past, More

All of the following statements are clues to past, more severe droughts that occurred in the Great Plains of North America EXCEPT:

A) salt- dependant diatoms are found in lake- sediment cores.
B) tree rings record evidence of past dry spells.
C) evidence of forest expansion from river valleys.
D) there is evidence of periods when sand dunes were expanding.