Which line helps understand both outer boundary and inner form of an object?

Correct answer: Contour and cross contour lines

The correct answer is A: contour and cross-contour lines work together to explain an object’s outside boundary and inner form. A contour line follows the outer edge of an object, helping the viewer identify its silhouette and limits. A cross-contour line travels across the surface, following its turning, roundness, thickness, or direction. For example, curved cross-contours on a cylinder can show that the surface wraps around rather than remaining flat. Option A is correct because it includes both kinds of information. Option B is wrong because a single dot gives almost no boundary or surface information. Option C is wrong because empty space surrounds or separates objects but does not describe their surface form by itself. Option D is wrong because a frame line marks the edge of the picture, not the changing surface of the object. Cross-contours are guides and may be visible or lightly suggested in a finished drawing. Memory cue: contour goes around the edge; cross-contour travels across the form.