What information raises concerns about Source B’s usefulness as evidence as an accurate depiction of slavery in 18th century?

I'm pretty sure it's c. extremely gradual but not 100% sure though.

Historians are researchers of the past or people who understand the history of people. with this field, they must possess qualities such as being unbiased, patient, efficient reader, has a knack for logic and critical thinking and reasoning, disciplined, open-minded, resourceful, with good analytical ability, good reasoning skills and problem solving!