Answer: D agglomeration effects
Explanation: D agglomeration effects is the main reason why the high-tech firms of Silicon Valley in California are clustered together. Agglomeration refers to the concentration of related industries or activities in a specific geographic location. This clustering can lead to positive externalities such as the sharing of knowledge, access to skilled workers, and increased innovation. By being located close to each other, firms in Silicon Valley are able to collaborate and exchange ideas, which can lead to increased competitiveness and productivity. This is why many high-tech firms choose to locate themselves in the Silicon Valley region, despite the high cost of living and doing business there.