Answer: B. American Indians had not been exposed to diseases from different parts of the world.
Explanation: At a time when Europeans transmitted their diseases to American Indians by settling America, it was not usual to travel often, except for special expeditions, researchers, invaders, occasional periodic displacement, etc. This meant that the inhabitants on one of the continents could for generations not come into contact with the inhabitants of other continents and civilizations. This again means that such peoples did not come in contact with their diseases and were not resistant to them. In order to achieve resistance to a disease, it is necessary that in the human organism there are antibodies that protect the body from these diseases, and this is achieved when they are in contact with these diseases and overtake them.