Black iron pipe is commonly used for which of the following applications?

Black iron pipe is commonly used for steam heating, gas, and air pressure piping, as well as hot water heating due to its durability and ability to withstand high pressures and temperatures. This type of pipe is specifically designed to handle the rigorous demands of these applications where both temperature and pressure fluctuations can occur. The material is resistant to corrosion and has a higher tensile strength than many other piping materials, which makes it suitable for carrying gases and liquids that require a robust delivery system. Additionally, black iron pipe does not need to be coated for use in these applications, since it is already manufactured with a protective layer that helps reduce corrosion from the elements involved in steam and gas transmission.

In contrast, the other options involve materials and applications that do not utilize black iron pipe. Water supply systems might use different types of materials such as PVC or copper for their respective advantages in corrosion resistance and flexibility. Electrical conduits must be made of non-corrosive materials and typically use PVC, aluminum, or galvanized steel. Ventilation ducts are made from materials that are lightweight and able to insulate against air leakage, which are not characteristics of black iron pipe. Thus, the proper use of black iron pipe in heating and gas applications distinguishes it as the correct choice.