why is it so difficult for historians to put an exact date on when the cognitive revolution began, and decide whether it should apply to other human species?

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.

d. hip hop

it was originated in the 70s while the harlem renaissance occured in the 1920s

the answer is d. selective incorporation. (apex)