Flexible talent introduction: It refers to adapting to the requirements for the socialization of talents, breaking the constraints in the flow of talents such as nationality, household registration, status, and personnel relations, and adapting to the market economy and the socialization of talents without changing and affecting the personnel relations between the talents and their units The required government guidance, market regulation, contract management of talent recruitment methods for non-local household registration talents. Which of the following are the flexible introduction of talents? ['In order to further improve the construction level and strength of a university, Professor Chen of Peking University is hired as an adjunct professor. Professor Chen will come to the university to teach every semester.', 'Professor Zhang is an expert at a university. When the computer school of the university applied for a national experimental base project, it also introduced the research results of the professor.', 'Professor C. Li is a well-known expert at home and abroad. A school has vigorously introduced Professor Li, not only arranging the work of his family members, but also solving the work problems of his children.', 'In the financial turmoil in 2008, Mr. Sun gave up his job in the United States and returned to Beijing to become an executive of a department store group.']