19. The fact that white light is light composed of various wavelengths may be demonstrating by
A B C
dispersing a beam of such light through a prism.
D
20. Over the course of history, much civilizations developed their own number systems.
A B C D
21. In the United States during the Second World War, each trade unions and employers avoided federal
A B
limits on wages by offering employees nontaxable medical benefits.
C D