The correct answer to this question is B. In HVAC (heating, ventilation, and air conditioning) there are two shapes of vertical ductwork: round and rectangular. For sheet metal ductwork, specified distances between hangers are given. With round vertical ductwork, the distance between hangers should not exceed 12 feet.
For rectangular vertical ductwork, the maximum distance between hangers is 10 feet. These distances were calculated based on the support needed to avoid distortion, buckling, and stress. Industry standards have been created and set at the international level, as well as in many individual countries around the world.