Why Were Native Americans Spared the Effects of Most of the World's

Why were Native Americans spared the effects of most of the world's communicable diseases until contact with the Europeans in 1492?

A) Until that time, they had eaten a nutrient-rich diet.
B) Until that time, they had not domesticated any animals.
C) Until that time, they did had not lived in large groups with close contact.
D) Until that time, they had rarely traveled out of their own communities.