Test analysis: This question examines students' understanding and memory ability of calligraphy development. In the Qin dynasty, seal script was the main feature of calligraphy art, and the strokes were round and even. So choose A. Official script appeared in Qin dynasty, but it did not become the main font; Regular script appeared in the Eastern Han Dynasty and spread in Wei and Jin Dynasties. Cursive script also appeared in the Han Dynasty. So BCD doesn't fit the question.

Comments: Calligraphy is an integral part of China's traditional culture and a treasure in China's art palace. With the proper density of Chinese characters, writing has aesthetic feeling. During the Wei, Jin, Southern and Northern Dynasties, it developed into a conscious art, and related knowledge points, such as the great calligraphers in ancient China, such as Wang Xizhi, Zhang Xu and Huai Su, also need to master their achievements.