All three statements regarding furnace oil are correct. Firstly, furnace oil is a byproduct of the oil refining process, making it a derivative of crude oil processing. Secondly, many industries, such as manufacturing and power generation, utilize furnace oil as a fuel source because of its high calorific value. Lastly, the combustion of furnace oil leads to significant sulfur emissions, contributing to air pollution and environmental concerns, particularly related to acid rain and respiratory issues. Understanding these points is essential for appreciating the implications of fuel choices on environmental sustainability in industrial operations, which aligns with UPSC's emphasis on the intersection of environmental science and public policy.