What does John Green say is what makes slavery so different in the Americas?

Forced slaves to work in fields and start the Civil War in America.

Explanation:

Slavery was hugely important to America which led to the Civil War. It lasted in U.S. history from 1619 to 1865. The South's economy was based on slavery. The South involved in the large cotton plantation which produces cotton for industries in the North and Britain. The profitability of slaved-based agriculture, meant the South to remain largely as an agricultural and rural state. Slaves forced to work in the fields with no contract sign for releasing them.

the answer is a. apartheid.