单项选择题

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"

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单项选择题

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.

单项选择题

This passage is （）

A．a short story
B．a popular science reading
C．a common knowledge in routine life
D．an essay