17) is the use of a light source to create highlights and shadows.

There's a good reason why - outside of a nice piece of flat design - nobody looks at 2D colored images twice. Anybody can do that. A quick Google search will prove it and you'll even wonder why you wasted your time scrolling through the bad examples. On the flip side, adding shadows and highlights to your work is going to leave a seriously good impression. So, here's how shadows and highlights are key to making work look professional.

where is the passage

i cant solve it without knowing the passage

it is based on the same philosophy as health insurance