The correct answer is B) Drum level and steam flow. In a steam boiler with a three-element feed water control, the two dominant control inputs are the drum level and the steam flow. The drum level is a critical parameter as it reflects the amount of water in the boiler, and maintaining the proper water level is essential for safe and efficient operation. The steam flow is also a key input because it represents the demand on the boiler, and the feed water flow needs to be adjusted accordingly to maintain the desired drum level. The other options are incorrect because feed water flow alone is not a dominant input (option C), and all three parameters do not have an equal effect on the output (option D). Option A is incorrect because steam flow, not feed water flow, is the dominant input along with drum level.