Correct Option: D

Human capital flight, more commonly referred to as “brain drain”, is the large-scale emigration of a large group of individuals with technical skills or knowledge. The reasons usually include two aspects which respectively come from countries and individuals. In terms of countries, the reasons may be social environment (in source countries: lack of opportunities, political instability, economic depression, health risks, etc.; in host countries: rich opportunities, political stability and freedom, developed economy, better living conditions, etc.).