Bigamy: refers to the fact that men and women with a spouse who have not completed the divorce procedure and registered the marriage with others, or live together with others without registration, constitute a de facto marriage relationship, and unmarried men and women who know that others have a spouse and marry them. According to the above definition, is the following behavior a bigamy? ['A and B voluntarily resumed husband and wife relationship after divorce and cohabited.', 'A gets a marriage certificate with unmarried B during the divorce.', 'A registers with unmarried B and has a son after divorce.', 'A marries B without knowing B is married.']