What problem may occur if straight parallel lines are placed on a round object?

Illusion of roundness and volume may weaken

Lines used on a rounded object should generally respond to the turning surface. Curved or contour-following marks help the viewer understand how the surface bends around the form, while straight parallel lines can appear pasted across it or ignore its curvature. Consequently, the illusion of roundness and three-dimensional volume may weaken, making option B correct. Such lines do not automatically improve the drawing, remove line weight, or make the object more natural; those claims in options A, C, and D are unsupported. The exact line direction depends on the object and the intended texture, but the governing principle is consistent: marks should support the observed form. Careful contour lines, varied pressure, and appropriate spacing can make a sphere, cylinder, or other rounded object appear more solid.

This is a Class 9 Art Education question. Chapter: Elements of Art. Topic: Line.