The correct answer is B) Multiple thermocouples can be used if they are wired in parallel, where the sum of the individual voltages is divided by the number of thermocouples. This is the correct answer because wiring multiple thermocouples in parallel and averaging their output voltages provides a signal that is proportional to the average temperature across the large duct. When thermocouples are connected in parallel, their individual voltage outputs are added, and then the sum is divided by the number of thermocouples to obtain the average. This technique effectively integrates the temperature variations across the duct cross-section, resulting in a signal that represents the true average temperature. The other options are incorrect because series-aiding or series-opposing connections would not produce an average signal, and using only one thermocouple would not accurately represent the average temperature across the large duct.