01 What is necessary to make a sphere appear three-dimensional?
Answer and explanation
Correct answer: A. Gradual light and dark shading
Explanation: The governing concept is tonal modeling: a solid form is shown by relating light, middle tone, and shadow. When light falls on a sphere, the illuminated area remains light, the opposite area becomes darker, and the transition between them is gradual. This produces the visual impression of curved surface and volume, so option A is correct. A circle or thick outline alone is flat, while an equal tone hides the form.