Why is not only density but also direction important in hatching?

Because direction suggests surface curve and volume

Hatching is a drawing technique in which repeated lines are used to create tone, shading, and a sense of three-dimensional form. The density of the lines controls how dark or light an area appears: closely spaced strokes usually look darker, while widely spaced strokes look lighter. Direction contributes different information rather than simply changing tone.

When hatching lines follow the curve or main direction of a surface, they help the viewer understand whether the form is rounded, flat, cylindrical, or bent. For example, curved strokes across a ball can suggest its round volume, while straight strokes may make a surface appear flatter. Thus direction suggests surface curve and volume, so option A is correct. It does not change the paper’s colour or erase the drawn lines.

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