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A duct heater is specifically designed to provide supplemental heat to air that is circulating through a duct system, which is typically part of a larger heating, ventilation, and air conditioning (HVAC) system. The key feature of a duct heater is its use of resistive heating elements, which generate heat by passing an electric current through a conductor. When air passes over these heating elements within the duct, it warms up before being distributed throughout the space.

This distinct function differentiates duct heaters from other systems. For instance, systems that use chilled water are focused on cooling, while duct heaters are dedicated to heating. Additionally, while many duct heaters are electric, the option does not capture the essence of their construction and operation, as duct heaters can also be configured to use other energy sources, such as natural gas or hydronic heating. Furthermore, portable heaters are not integrated into duct systems but rather function independently in small or specific areas.

In summary, the defining characteristic of a duct heater is indeed the presence of resistive heating elements within a duct, making option B the most accurate description.