You are correct, the answer D is the correct explanation for why double flow reaction turbines produce very little axial thrust. In a double flow reaction turbine, the steam flow is split into two equal streams that flow in opposite directions through the turbine. This results in the axial thrust developed on the rotor in opposite directions, effectively canceling each other out and producing very little net axial thrust. The other options are incorrect because: A) the pressure drop across the blades is still present, B) the equalizing holes are not the primary reason for the low axial thrust, and C) the steam expansion across the entire turbine, not just the low-pressure stage, contributes to the reduced axial thrust.