单选题 .  ①Early in the twentieth century, San Francisco was the main venue for African American jazz musicians on the West Coast of the United States. ②Musical activity was centered in a district known as the Barbary Coast, where an abundance of nightclubs provided ample work opportunities for local players and drew musicians and other entertainers, many of them African American, from throughout the country. ③In 1921, as part of its Prohibition-era efforts, the government closed the Barbary Coast. ④This closure was the decisive event that established Los Angeles as the premier center for jazz on the West Coast. ⑤Once the Barbary Coast was shut down, it became far harder for jazz musicians to make a living in San Francisco; thus, many headed south to Los Angeles.
    ①Yet even before that closing, the center of jazz activity had begun to swing southward. ②With the largest and fastest growing African American urban community in the West, as well as the growing movie industry and an emerging recording industry, Los Angeles was already a magnet for jazz musicians from other parts of the country, especially New Orleans, where jazz players suffered a devastating blow with closing of the Storyville district in 1917.13.  The primary purpose of the passage is to ______