单项选择题
A．a material that can be seen through B．a translucent(半透明的) material C．a material that can not be seen through D．a lucent material
A．"Why are there shadows" B．"The transparent materials" C．"How to cast a shadow" D．"Shadows and light waves"
A．Light waves are beams of light. B．Light waves can penetrate any materials except men' s bodies. C．Light waves can penetrate transparent materials. D．Light waves can penetrate some materials.