Hidden employment: refers to a job status that is not obtained through the traditional employment channels and is outside the scope of employment statistics of relevant departments. 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 guide lectures outside of work.', 'Xiao Chen provides platform building and maintenance services for e-commerce stores after leaving the company.', 'Wang, who loves writing, became a freelance copywriter for a newspaper after graduating from college.']