Which of the Following Best Describes What Allows Slavery to Exist

Which of the following best describes what allows slavery to exist in today's world?

A) Some countries do not have laws against the practice of slavery.
B) Some countries actually sanction slavery as a legal form of work; much like how slavery was viewed in the South before the Civil War.
C) Slavery is carried out surreptitiously for the most part, and can sometimes be difficult to identify by law enforcement.
D) The United Nations has allowed some countries to practice slavery for a few years until it can be phased out.