Which of the Following Best Explains the Lack of Resistance

Which of the following best explains the lack of resistance of Native Americans to European diseases?

A) Most Native Americans had never done hard labor before Europeans arrived and were not used to the stress.
B) Most Native Americans were unaware of the relationship between disease and sanitation.
C) Most Native Americans did not understand the germ theory of disease and so could not take preventive measures.
D) Native America lacked the agriculture and industrialization that fosters disease formation.
E) Native America lacked the domestic animals that were the original sources of many diseases.