Five doors. Mathematics, Chinese, music, pictures, sports and nature. In 1970s, there was no nine-year compulsory education in China, and primary schools only ended from grade one to grade five. In the first, second and third grades of primary school, there are only five simple courses: mathematics, Chinese, music, painting and physical education. From grade four to grade five, natural courses are added on the basis of the above courses, so that students can understand the knowledge in life by learning some natural phenomena in nature.