题目内容：

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

2. They have a variety of purposes. The cement (水泥) industry uses revolving kilns to heat and 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 （在窑里烘干的）before they can be used. Hobbyists use small kilns to fire 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 raised to a high temperature and then completely cooled before its contents are removed. A continuous, or tunnel kiln contains several chambers, or zones, which heat, bake, and cool a product. Stacks of clay bricks move through a tunnel kiln on tracks. 3.

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

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