The knowledge points to be examined in analyzing this question are the subject of patent application right. This knowledge point is now the test site in the Practice of Enterprise Legal Counsel. According to Article 6 of the Patent Law: "The right to apply for a patent for a service invention-creation belongs to the entity; The right to apply for a patent for a non-service invention-creation belongs to the inventor or designer. " Of course, the patent application right of the valve developed by A factory belongs to A factory. Although Qi is an engineer in the second factory, Qi is a self-determined project, purchasing equipment and scientific research materials, and independently completing the development of the valve in his spare time, which is not a job invention, and the right to apply for a patent belongs to Qi himself. Therefore, the engineers of Factory A and Factory B have the right to apply for patents. Therefore, the correct answer to this question is AC.