Annealing is a way of making metal softer by heating it and then letting it cool very slowly. If metal is heated and then cooled very quickly, for example by dipping (浸) it in water, it will be very hard but also very brittle (脆) —that is , it will break easily. Metal that has been annealed is soft but does not break as easily. It is possible to make metal as hard or as soft as is wished, by annealing it. The metal is heated, and allowed to cool slowly for a certain length of time. The longer the heated metal takes to cool slowly, the softer it becomes. Annealing can also be used on other materials, such as glass.

小题1: Annealing can make metal      

A．hard and tough (韧).

B．hard but brittle.

C．soft but tough.

D．soft and brittle.小题2:Why do people put hot metal in water?

A．To make it hard.

B．To make it soft.

C．To make it cool.

D．To make it brittle.小题3:As suggested by the text, how can glass be made less brittle?

A．It can be heated and then cooled quickly.

B．It can be cooled and then heated slowly.

C．It can be heated and then cooled slowly.

D．It can be cooled and then heated quickly.

VD炉抽气系统的切换阀一般采用（）阀。