Which of the Following Is an Example of "Brain Drain

Which of the following is an example of "brain drain"?

A) An American university professor is hired as a part-time consultant by a developing nation to assist with economic development strategies.
B) Test scores are falling in a developing nation that has had no choice but to significantly reduce its expenditures on education.
C) A young engineer from a developing nation leaves his native country to take a high-paying job in an industrialized nation.
D) The best teachers in a developing nation are hired by the country's government for nonteaching tasks at a salary higher than the schools can pay.