Why is cross-contour useful in understanding the relation between line and surface?

Correct answer: It guides the line along the direction and curvature of the surface

Direct answer: A, because a cross-contour line follows the direction and curvature of a surface. An outer contour describes the edge or silhouette of an object. A cross-contour, in contrast, travels across the object as though it were wrapping around or passing over its surface. To use it, first imagine the form as a three-dimensional object. Next, observe whether the surface is flat, rounded, bulging, dipping, or changing direction. Then draw lines that bend or turn in agreement with that surface. These lines can help the viewer understand roundness, volume, orientation, and surface movement. A is correct because it states this relationship directly. B is wrong because it describes an outer contour, not a cross-contour. C is wrong because cross-contours usually add information rather than hide it. D is wrong because they are drawn marks and do not themselves change colour. Cross-contours do not magically create a solid form; they are a visual aid whose success depends on following the form consistently. Memory cue: outer contour goes around the edge; cross-contour goes across the surface.