In a large sky area, the colour unintentionally became lighter toward the bottom. If gradation was not intended, what may be the cause?

Correct answer: The brush was not reloaded with the same colour mixture while moving downward

The governing concept is consistent pigment concentration for an even fill. With repeated strokes, the brush gradually loses pigment and water may dominate, so later strokes become lighter if it is not reloaded with the same mixture. Option B is correct. A blue sheet, painting the upper part first, or slight tilt does not by itself explain a systematic loss of colour strength toward the bottom.