Non-ohmic behavior of filament lamps due to temperature-dependent resistance

Steps:

• At low V, the filament is cool with low resistance, so I rises quickly with V.
• As V increases, the filament heats up, raising its temperature.
• Higher temperature increases resistance, reducing the rate of I increase.
• The I-V graph thus shows a curve that starts steep and flattens, not linear.

Why A is correct:

• A depicts the non-linear I-V curve where resistance rises with temperature, following I = V/R with R increasing (from heating effect in metals).

Why the others are wrong:

• B shows a straight line, implying constant resistance (ohmic conductor), not true for lamps.
• C likely shows a linear increase but steeper, ignoring resistance change.
• D probably shows an exponential rise, like semiconductors, not metallic filaments.

Final answer: A