The Following Questions Refer to the Excerpt Below

The following questions refer to the excerpt below.
"The presence of explorers such as Verrazano and Cartier and of unknown numbers of anonymous fishermen and part-time traders had several effects on the native population.The Mi'kmaqs,Hurons,and other northeastern Indian groups approached the intruding Europeans in friendship,eager to trade and to learn more about the strangers.In part this response was a sign of natural curiosity,but it also reflected some serious changes taking place in the Native world of North America.Beginning in the mid-fourteenth century,the climate in North America underwent significant cooling.As the climate grew colder,both hunter-gatherers like the Mi'kmaqs and agriculturists like the Iroquois had to expand their subsistence territory,and in doing so they came into conflict with their neighbors.As warfare became more common,groups increasingly formed alliances for mutual defense-systems like the Iroquois League and Powhatan Confederacy."
Historian Christopher L.Miller,"The Americas to 1620"
-One factor which led to the changes described in the passage was

A) decreasing natural resources in North America, which led to an increasingly migratory culture.
B) decreased warfare in the northwest among native tribes.
C) decreasing social diversification resulting from maize cultivation.
D) the introduction of African slaves into the Americas.