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The History of Chalk
Chalk used in school classrooms comes in sticks . Lessons are often presented to entire classes on blackboards using sticks of chalk. ___26 ___
Chalk has been used for drawing since ancient times.___27___ Later, artists of different countries and styles used chalk mainly for sketches（素描）.
Chalk did not become standard in schoolrooms until the 19th century.___28 ___ Teachers needed a convenient way of teaching many students at one time. Instructors used large blackboards, and students also worked with individual chalkboards.
Blackboards used to be black，because they were made from real slate（石板）.Some experts made yellow chalkboards and dark blue chalk to simulate（模拟）writing on paper. However, when manufacturers began to make chalkboards from man-made materials during the 20th century, they chose the color green.__29____ Later ,yellow became the preferred color for chalk.
Almost all chalk produced today is dustless, Earlier, softer chalk tended to produce a cloud of dust. ___30___ Dustless chalk still produces dust; it's just that the dust settles faster. Manufacturers accomplish this by baking their chalk longer to harden it more.


A. The manufacturing of chalk is becoming more complicated.
B. Some feared such dust might cause breathing problems.
C. It helped to create some of the earliest cave drawings.
D. They argued that it was easier on the eyes.
E. At that time class sizes began to increase.
F. This method has proven cheap and easy.