Look at a man walking down a street or road in' the sunlight and you will see his shadow.

If you watch two men walking in the sunlight and carrying a six-fool pane(窗玻璃片) of glass between them, you will see each man six feet apart, but there will be no shadow of the pane of glass.

Shadows are easily explained since we know that light travels in a straight line. We also know that light waves pass through some bodies and not through others.

When light strikes a body and passes through it unchanged, we call such a body or material transparent. Most glass is such a material, especially window glass, and it is for this reason that we do not see the shadow of the window pane which the men are carrying. Do you know of any other transparent materials？ The most common one, with which we are familiar, is air. Another is clear water.

On the other hand, some materials do not let light pass through them at all. They stop the light waves just as you would-catch a ball that was being thrown to someone standing between you. Such materials are called opaque. Men' s bodies, like steel, rock, concrete or even cardboard, are opaque and do not permit the light to pass through. An opaque body casts a shadow.

The title that best expresses the ideas of the passage is ______.

A．Why are there shadows

B．The transparent materials

C．How to cast a shadow

D．Shadows and light waves