Why did tenant farming become a dominant form of agriculture in the 1870s?

What does the author describe in this passage

Which statement about mound builder works is true? earthen mounds were created for children to play on. bowls shaped like animals were made of stone. portraits were made of solid gold and painted with bright colors

According to the federal regulations, human subjects are living individuals about whom an investigator conducting research obtains information through interaction or intervention with the individual, and uses, studies, or analyzes the information; or: