What is the evidence Hayden uses to argue that the large pithouses were divided into two separate groups in The Pithouses of Keatley Creek?
a) Different artifact distributions and architectural features suggest two distinct groups within the pithouses.
b) Oral histories from the region confirm the division of pithouses into two groups.
c) Carbon dating indicates two distinct time periods for the construction of pithouses.
d) Analysis of pollen samples inside the pithouses shows different floral preferences among the inhabitants.