Answer: d. all of the above
Explanation:
The Forbes 400 is a an annual list published by Forbes magazine that shows the richest 400 people in the United States.
The Wealthy 100 is a book published in 1996 that showed the richest Americans of all time.
The Rich and the Super-Rich is a book by Ferdinand Lundberg that was published in 1968 and documented 500 wealthy American families who were said to run the economy.
All the above have therefore studied and documented the wealthy.
