Final answer:
The alcohol trade in St.Pierre resulted in the production of molasses as a by-product, which was key to the triangular trade linking the Caribbean, the American colonies, and Africa.
Step-by-step explanation:
The helpful by-product produced by the alcohol trade in St.Pierre was molasses, which is a by-product of sugar production. The plantation owners in the Caribbean who grew sugar cane would ship molasses to Massachusetts, where it was transformed into rum.
This rum was then shipped to England or exchanged in African ports for captives, creating a cyclical trade, often referred to as the triangular trade. This sequence indicates how alcohol production was intricately linked with other industries and sustained numerous businesses and labor markets.