Questions: What is true of cross contour lines? They are invisible lines that add form to objects. They can be seen on all objects. They can only be drawn on paper.

The answer is the first one: They are invisible lines that add form to objects.

Explanation for each option:

1. They are invisible lines that add form to objects.
   This statement is correct. Cross contour lines are used in drawing to suggest the three-dimensional form of an object. They follow the surface of the object, helping to convey its volume and shape, even though they are not actually visible on the object itself.

2. They can be seen on all objects.
   This statement is incorrect. Cross contour lines are not visible on objects in real life. They are a drawing technique used to represent the form and volume of an object on a two-dimensional surface.

3. They can only be drawn on paper.
   This statement is incorrect. While cross contour lines are often drawn on paper, they can be created on any medium suitable for drawing, such as canvas, digital screens, or any other surface where artists can depict form and volume.

In summary, cross contour lines are a drawing technique used to add form and volume to objects, and they are not visible on the objects themselves.