Why is alternating-current (AC) power transmission more effective than direct-current (DC) transmission over long distances in conventional grids?

Introduction:
Power loss during transmission is predominantly resistive, scaling with the square of current. This question tests understanding of why legacy AC grids efficiently move power over long distances using transformers.

Given Data / Assumptions:

• Line resistance causes heating losses approximately equal to I^2 * R.
• Transformers operate on AC and allow voltage conversion with high efficiency.
• Total transmitted power P ≈ V * I (ignoring reactive effects for simplicity).

Concept / Approach:
For a given power P, increasing transmission voltage V allows current I = P / V to decrease. Since resistive loss is I^2 * R, raising V and lowering I dramatically reduces losses. AC enables practical use of transformers for stepping voltage up (transmission) and down (distribution), which is the traditional reason for AC's advantage.

Step-by-Step Solution:
1) Start with P = V * I for the same required delivered power.2) Increase V via a step-up transformer so I = P / V becomes small.3) Line loss reduces as I^2 * R; with smaller I, losses drop quadratically.4) Near loads, use step-down transformers to safely reduce V and raise I as needed.

Verification / Alternative check:
Compare two scenarios moving 100 MW: at 100 kV the current is 1,000 A; at 500 kV it is 200 A. Loss ratio scales roughly as (200/1000)^2 = 1/25—showing the dramatic reduction.

Why Other Options Are Wrong:
Height of power lines: affects clearance and safety, not fundamental efficiency.

Use of AC generators: a historical fact but not the efficiency reason.

Very high voltages: true but incomplete—transformers (enabled by AC) make this practical and safe; option (c) states the mechanism.

Zero resistance: incorrect; lines have finite resistance.

Common Pitfalls:
Confusing the outcome (high voltage) with the enabling mechanism (transformers on AC). Modern HVDC also achieves low loss but uses power electronics; the question targets the classical AC advantage.

Final Answer:
due to step-up and step-down transformers reducing I^2R losses