After the discovery of America during the 1400s started the Columbian exchange between the old and the New World. Europeans introduced domestic animals and some crops along with diseases that killed the entire civilizations of the Native Indians after they came into contact with the Europeans. Some of the diseases included smallpox, chickenpox, malaria, measles, cholera, scarlet fever, etc.
It certainly wasn't C and D, however critics of US foreign policy would possibly cite A. I would argue in the context of the question it is definitely B.
