TOEFL Listening: TPO-TOEFL阅读TPO - UY5492E839IH2EHTN$

Which of the following allowed Teotihuacán to have "a competitive edge over its neighbors"? A. A well-exploited and readily available commodity B. The presence of a highly stable elite class C. Knowledge derived directly from the Olmecs about the art of toolmaking D. Scarce natural resources in nearby areas such as those located in what are now the Guatemalan and Mexican highlands