Concept:Fish farming is best combined with enterprises that exchange useful by-products or resources.
Explanation:Vegetable production benefits from nutrient-rich fish waste as fertilizer.
The vegetables, in turn, help purify the water for the fish.
Poultry droppings can serve as food for fish, and fish waste can be used in poultry feed.
Beekeeping mainly depends on flowering plants and does not share a direct by-product exchange with fish farming.
Therefore, the pairings that give mutual benefits are vegetable production and poultry production only.
Answer:The correct option is A. I and III only.