According to the Case Study on the Traffickers Who Bring

According to the case study on the traffickers who bring illegal workers to high-wage countries such as the United States, the United Kingdom, or Germany, these intermediaries are able to carry out their lucrative trade because:

A) the wage differences between places like Europe and China represent huge arbitrage opportunities.
B) there are no restrictions on immigration by the developed countries.
C) there are many legal ways for immigrants from poor countries to travel to Europe and other high-income areas as tourists or students.
D) All of the above.
E) None of the above.