As per Wikipedia's History of Kolkata page:
After the territorial conquest of Bengal in 1757, the British East India Company pursued a monopoly on production and export of opium from India. The company bought opium from local traders and later directly from farmers, and sold it at auction in Calcutta. From there much of it was smuggled to Canton in China by foreign traders, eventually leading to the First Opium War (1839–1842).
Given that Calcutta was not anywhere near Canton and there weren't any direct routes (or were there?) from Calcutta to Canton, why was Calcutta chosen for those auctions instead of some place closer? Was it simply because it was under British control or was there any other reason?
Also, what routes did these smugglers take to smuggle opium from Calcutta to Canton?