Two parents purchased a new mobile home from a seller. the mobile home was manufactured by mobilco and had a ventilating system designed by mobilco with both a heating unit and an air conditioner. mobilco installed a furnace manufactured by heatco and an air conditioning unit manufactured by coolco. each was controlled by an independent thermostat installed by mobilco. because of the manner in which mobilco designed the ventilating system, the first time the ventilating system was operated by the parents, cold air was vented into the parents' bedroom to keep the temperature at 68 degrees f (20 degrees c). the cold air then activated the heater thermostat, and hot air was pumped into the bedroom of the six-month-old child of the parents. the temperature in the child's room reached more than 170 degrees f (77 degrees c) before the child's mother became aware of the condition and shut the system off manually. as a result, the child suffered permanent physical injury. claims have been asserted by the child, through a duly appointed guardian, against mobilco, the seller, heatco, and coolco. if the child's claim against the seller is based on negligence, the minimum proof necessary to establish the seller's liability is that the ventilating systema. was defective. b. was defective and had not been inspected by the seller. c. was defective and had been inspected by the seller, and the defect was not discovered. d. was defective, and the defect would have been discovered if the seller had exercised reasonable care in inspecting the system.