I'm pretty sure its A. America started the other ones you listed.
Answer:
A premium
Explanation:
A premium offers an item for free or at a bargain price to reward some type of behavior, such as buying, sampling, or testing.
I believe the answer is: the Columbian Exchange
Columbian exchange refers to the exchange of knowledge, labour culture, plants , and animals that heppened between countries in american continent during the 1500s. When the exchange happens, different races crossbreed with one another which resulted in the birth of new races.
They manipulated the length of time between seeing and recalling sets of 3-letter stimuli while preventing rehearsal.