Invisible employment: Refers to the work status that is outside the scope of employment statistics of relevant departments without obtaining a fixed occupation through traditional employment channels. Which of the following are not invisible employment? ['Yan resigned from work and started a milk powder online shop while taking his children.', 'A professor is invited by many training schools to teach in his spare time.', 'Xiao Chen provides platform building and maintenance services for e-commerce stores after leaving the company.', 'Wang, who loves writing, became a freelance writer for a newspaper after graduating from college.']