Why may patches form while shading with circular strokes?

Correct answer: Because pressure and stroke density are uneven

The governing concept is even distribution of tone. Circular-stroke shading becomes patchy when the artist presses harder in some areas or places more overlapping strokes there than elsewhere. Those differences create uneven graphite density, so the eye sees separate dark and light patches. Option C is correct. White paper and a light contour are not causes of uneven fill, and a sharp point may affect line character but does not by itself create patches when pressure and density are controlled.