Progressive retirement: Refers to the employees reaching the legal retirement age, and the employer flexibly determines the retirement time for some employees according to the actual work needs. Which of the following are progressive retirements? ['Professor Ma is approaching retirement age because he is one of the most influential scholars in China and has made a significant contribution to the development of the subject. The school decided to hire him as a tenured professor.', 'Mr. Zhao has reached the retirement age, but the major research project he is hosting has not yet been resolved, and his research institute has decided to hire him to continue working until the project is completed.', "In recent years, Engineer Li's physical condition has become worse and worse, and he has suffered from mild depression and is unable to continue to work. After research, the company leadership decided to let him retire early.", 'A research institution decides that starting from the second half of the year, employees who apply for early retirement in this unit shall have a minimum of 54 years of primary and intermediate titles and a minimum of 58 years of senior titles.']