Read the text and answer the question. The construction of the Great Wall of China is one of the most ambitious engineering feats in human history. It began as early as the 7th century BC, with various sections built by different Chinese states to protect against nomadic invaders. The most famous sections were constructed during the Ming Dynasty (1368-1644 AD), which significantly extended and reinforced the wall. Its primary purpose was military defense, but it also served as a border control, allowing for the imposition of duties on goods transported along the Silk Road. The wall's construction involved millions of laborers over centuries, often under harsh conditions. What was one of the main purposes of the Great Wall of China, besides military defense?
A) To serve as a trade route for merchants.