According to paragraph 1, which of the following is true of the artwork of the Magdalenians? A. Its best features were later made part of a unified art system by other people after the end of the Paleolithic period. B. It includes the finest examples of paintings done on rocks. C. Compared to the artwork of later groups, it was produced for only a short time. D. Its earliest forms were on open-air rocks, while its later forms were in caves.