题目内容：

    Chalk used in school classrooms comes in thin sticks. Lessons are often presented to entire classes on chalkboards (or blackboards, as they were originally called) using sticks of chalk. 1.

As found in nature, chalk has been used for drawing since prehistoric times. And it helped to create some of the earliest cave drawings. Later, artists of different countries and styles used chalk mainly for sketches(素描),and some such drawings have survived. 2.The method was to grind(碾碎)natural chalk to a fine powder, then add water, clay, and various dry colors. It was then rolled into stick shape and dried.

3. Class sizes began to increase at that time. Therefore, teachers needed a convenient way of conveying information to many students at one time. Not only did instructors use large blackboards, but students also worked with personal chalkboards, completing with chalk sticks and a sponge or cloth to use as an eraser. These small chalkboards were used for practice, especially among the younger students.

An important change in the nature of classroom chalk brought was in chalkboards. Blackboards used to be black, because they were made from true slate(石板).While some experts advocated a change to yellow chalkboards and dark blue or purple chalk to copy writing on paper, when makers began to shape chalkboards from synthetic(合成的)materials during the 20th century, they chose the color green, arguing that it was easier on the eyes. 4.

Almost all chalk produced today is dustless. Earlier, softer chalk tended to produce a cloud of dust that some feared might contribute to breathing problems.5. It's just that the dust settles faster.

A. Dustless chalk still produces dust.

B. Yellow became the preferred color for chalk.

C. Paper made only from rags at this time was expensive.

D. That's because this method has been proven cheap and easy.

E. Chalk was first formed into sticks for the convenience of artists.

F. Baking chalk longer to harden it more helps to produce less dust.

G. Chalk didn't become standard in schoolrooms until the 19th century.