题目内容：

A kiln (窑) is a type of oven or furnace used to cure, harden, or burn certain materials. Depending onwhich materials are heated in it, a kiln may be square, tunnel shaped, or beehive shaped, and usually madeof brick or stone.    1. It may be set as low as sixty-five degrees to cure grain or as high as 3,200degrees to transform limestone to quicklime (熟石灰).

2.They have a variety of purposes. The cement(水泥) industry uses revolving kilns to heatand separate raw materials. Bricks, pottery, and china made from clay must be fired, or hardened, in kilns.Certain kinds of coal and wood must be kiln-dried before they can be used. Hobbyists use small kilns tofire a variety of ceramic(陶瓷) objects.

There are two main types of kilns. A periodic kiln, used to fire certain kinds of delicate china, is raisedto a high temperature and then completely cooled before its contents are removed. A continuous, or tunnelkiln contains several chambers, or zones, which heat, bake, and cool a product. Stacks of clay bricks movethrough a tunnel kiln on tracks.  3.

Gas, oil, and coal are all used to heat modern kilns.   4. But for special jobs, where cleanlinessand careful heat are important, kilns must be heated by electricity oven though it is more expensive. Smallkilns used in the home or in craft shops are also heated electrically because electricity is clean andconvenient.

5. When ancient people discovered that heat hardened some materials, they were able to makedurable household objects such as jugs and bowls. Today our museums display many kinds of ancientstatues and art objects that have lasted because they had been kiln-dried. They come from lands as far apartas China and Greece.

A. Its temperature may vary.

B. Then they come out, hardened, and cooled.

C. Kilns are used in industry and in craft-making.

D. Kilns have been in use for thousands of years.

E. An electric kiln is a heating chamber used to transform materials.

F. Most manufacturers use whichever material is cheapest in their area.

G. Clay, when heated properly, becomes hard enough to form bowls and plates.