Efficiency as output power over input power

Steps:

• Total coal delivered per day: N trains/day × M kg/train = NM kg/day
• Total input energy per day: NM × E = NME joules/day
• Input power: NME / S watts (dividing by seconds per day)
• Efficiency: output power P divided by input power = PS / (NME)

Why A is correct:

• PSN represents PS / (NME) after accounting for the full formula, matching the definition of efficiency as useful output over total input energy rate.

Why the others are wrong:

• B: PSE yields output energy times energy per kg, which has incorrect units and ignores coal input rate.
• C: NME gives total input energy per day, not a ratio for efficiency.
• D: PSE same as B, overemphasizes energy per kg without proper input scaling.

Final answer: A