Final answer:
Migration of the First Americans involved both slow and quick movements, depending on the routes and methods used, including coastal navigation and overland travel. Hence, the correct answer is d. Both slowly and quickly.
Step-by-step explanation:
After the First Americans arrived, migration methods and speeds varied. Some groups may have migrated down the west coast using a combination of land travel and coastal navigation by raft or canoe, which would have been relatively quick. However, due to rising sea levels that covered early coastal migration routes, solid evidence is scarce.
As for the interior routes, once past the ice sheets, humans spread into the continental United States reaching areas such as modern Mexico, Central America, and South America. The process of migration likely involved both slow and quick movements depending on geography and resources.
The Pacific coastal migration theory, supported by archaeological evidence, suggests that sea travel along the coastlines would have allowed for faster and easier movement, leading to quicker spread throughout the Americas. Conversely, the migration through the interior, between melting ice sheets, would have been more arduous and potentially slower.
Considering the varying routes and methods of migration, the best answer to the question of whether the First Americans migrated slowly or quickly to points south is d. Both slowly and quickly.