Answer:
There were five original Iroquois Nations; in the early 1700s, the Tuscarora joined the Confederacy as the sixth.
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Question 1: which. Question 2: that.
Since before recorded history, environmental changes have affected things people value. In consequence, people have migrated or changed their ways of living as polar ice advanced and retreated, endured crop failures or altered their crops when temperature and rainfall patterns changed, and made numerous other adjustments in individual and collective behavior. Until very recently, people have responded to global phenomena as if they were local, have not organized their responses as government policies, and have not been able to respond by deliberately altering the course of the global changes themselves. Things are different now from what they have been for millennia.
Plural verbs go with singular subjects (False), so (D) is your answer.
An example would be "The cat are fun."
