The letter C. There are 22 basic categories in library classification, which are represented by Chinese phonetic alphabet, as follows: A Marxism-Leninism, Mao Zedong Thought, Deng Xiaoping Theory, B Philosophy, Religion, C Social Sciences, D Politics, Law, E Military, F Economy, G Culture, Science, Education, Sports, H Language, Writing, I Literature, J Art, K History, and so on. P Astronomy, Earth Science, Q Biological Science, R Medicine, Health, S Agricultural Science, T Industrial Technology, U Transportation, V Aviation, Aerospace, X Environmental Science, Labor Protection Science (Safety Science), Z Comprehensive Books.