According to paragraph 4, what has recent research on obsidian tools found at Olmec sites shown? A. Obsidian's value was understood only when Teotihuacán became an important city. B. The residents of Teotihuacán were sophisticated toolmakers. C. The residents of Teotihuacán traded obsidian with the Olmecs as early as 400 B.C. D. Some of the obsidian used by the Olmecs came from the area around Teotihuacán.