Our first frontiers were the settlements along the Coast. atlantic pacific

Atlantic    

Explanation:

The term frontier described as a region at the end of a settled area. It began with the first days of European settlement on the Atlantic coast. In the 17th century, the frontier increased up the Atlantic Coast, which introduced to the settled area. It was the frontier of Europe in a real sense which combined with farmers, fisherman, fur-trader, artisans, miner, cattle-raiser. As the Atlantic frontier began to take place, we get to know about European life with the wilderness for survival. The first frontier had to meet its Indian question, with the increase of political and religious actions.

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.

d is the correct answer

~courtney