I think its A.
Sorry if it isn't.
The native Americans already had the first 3 items. The British brought horses to the new world and the native Americans used them a good bit Civil War and after.
An elector has a pledge to vote according to an agreement. If the elector failed to vote the candidate they pledge to vote, they are now called faithless elector. However, based on the study faithless electors are a few and had not affected the voting process. Still, this only shows that they don't keep their pledge and do not trust the candidate that they supposed to vote.
Answer:
The limbic system maturing before the prefrontal cortex is the correct answer.
Explanation: