Which of the Following Formed the Crux of Porter's Study

Which of the following formed the crux of Porter's study of national competitive advantage?

A) Identifying the various stages of the life cycle of a product
B) Determining why a country achieves international success in a particular industry
C) Determining how trade barriers affect the prices of products in the international market
D) Determining how pre-industrialization theories predict international trade patterns
E) Determining the relationship between factor endowments and economies of scale