Efficiency as output over input energy

Steps:

• Identify input energy as chemical energy W from fossil fuel.
• Identify useful output energy as electrical energy Y.
• Recall efficiency formula: useful output divided by total input.
• Apply formula: efficiency = Y / W.

Why A is correct:

• Efficiency is defined as useful energy output (Y) divided by total energy input (W), per the law of conservation of energy.

Why the others are wrong:

• B divides by W+X, including wasted thermal energy X, which overstates losses.
• C divides by X only, ignoring initial chemical input W.
• D divides by W+X+Y, including output Y in denominator, which is incorrect.

Final answer: A