阅读下面这篇短文，短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为第2～5段每段选择1个正确的小标题；(2)第27～30题要求从所给的6个选项中选择4个正确选项，分别完成每个句子。
Metallurgy
1．Metallurgy (治金学) is the art and science of making metals and alloys (合金) in forms and with properties suitable for practical use. The art of metallurgy includes the deriving of metals from their ores, or the condition in which they are found in nature; their purification (提纯) or their admixture with other metals, and finally their manufacture into shapes and forms usable in industry. The science of metallurgy includes the study of these processes with a view to (以便) their control and improvement, and the development of new metal mixtures or alloys and of new test methods.
2．Because of this wide scope the field of metallurgy may be divided into two parts. The first part deals with the melting (熔化) and refining (精炼) of metals, and has been designated as process of Chemical Metallurgy. The second deals with the physical and chemical behavior of the metals during shaping and treating operations, and their behavior in the service of man. This phase is termed Physical Metallurgy.
3．The scope of Physical Metallurgy is wide and is of interest to more people than the field of Process Metallurgy. For example, only a few individuals will be engaged in the operation of a melting and refining furnace, whereas a hundred men will be employed in the rolling (轧制) or forging (锻造) of the metals produced, and thousands of individuals will work at manufacturing the rolled or forged metal into automobiles, bridges, ships, airplanes, buildings, wire goods, tools, and a multitude of useful articles.
4．Everyone comes in contact with metals through his many everyday uses; therefore the term metal is well known. Not everyone realizes, however, why metals have come to play so large a part in man’s activities. Wood and stone are both older in use, yet to a considerable extent they have been substituted by the metals. The reason for the increased use of metals is to be found in their characteristic properties.
5．Most important of these properties is their strength (强度), or ability to support weight without bending or breaking, combined with toughness (韧性), or the ability to bend rather than break under a sudden blow. Resistance to atmospheric destruction, plasticity (弹性) ,and the ability to be formed into desired shapes add to the remarkeable combination of properties possessed by no other class of materials. Some metals also have special additional properties, two of which are the power to conduct electric current and the ability to be magnetized. The main properties are characteristic of their ______.