求大神给我做下这篇英语阅读理解，最好有解释

Question

A material is generally used because it offers the required strength, and other properties, at minimum cost. Appearance is also an important factor. The main advantage of metals is their strength and toughness. Concrete may be cheaper and is often used in building, but even concrete depends on its core of steel for strength.
Plastics are lighter and more corrosion-resistant(抗腐蚀的)，but they are not usually as strong. Another problem with plastics is what to do with them after use. Metal objects can often be broken down and the metals recycled; plastics can only be dumped or burned.
Not all metals are strong, however. Copper and aluminum, for example, are both fairly weak—but if they are mixed together, the result is an alloy(合金), which is much stronger than either pure copper or pure aluminum. Alloying is an important method of obtaining whatever special properties( 性能)are required: strength, toughness, resistance to wear, high electrical resistance or corrosion-resistance.
The properties of a metal can be further improved by use of heat treatment. Heat treatment is the term given to a number of different procedures in which the properties of metals and alloys are changed. It usually consists of heating the metal or alloy to a selected temperature below its melting point and then cooling it at a certain rate to obtain those required properties. For example, hardening is used to make metals harder. Tempering makes them softer and less brittle(硬而易碎的). Annealing is conducted to make a metal soft so that it can be machined more easily. In this way, metallic materials can be produced to meet every kind of engineering requirement.
31. When choosing a material, people may consider all the following except the material’ s ______.
A. Strength B. appearance
C. recyclability D. cost

32. Which of following statements is true?
A.Concrete must always be used together with steel in building.
B.Aluminum bronze is stronger than any pure metal.
C.A major advantage of plastics over metal is that they are cheaper than metal.
D.It’s sometimes an advantage for a metal to be soft.

33. Alloying is useful in that______.
A.It turns a weak metal into a strong one
B.It makes such useless metals as copper and aluminum useful
C.It produces new materials that can meet special requirements
D.All of the above

34. Which of the following methods can be used to obtain a more corrosion-resistant material?
A. Alloying. B. Hardening.
C. Tempering. D. Annealing.

35. Generally, what is heat treatment used for?
A.To improve the quality of a metal or alloy.
B.To obtain a special property of a metal or alloy.
C.To melt a metal or alloy to make it easier to use.
D.To make a metal or alloy stronger.

Answer 1

其中第33题比较难
第33题的b选项在文中并没有提到过，所以答案不可能是d
而a的意思是一种weak的材料变成一种strong的材料，但是根据文中，合金的意思是两种材料的结合，所以前后并不是同一种材料。因此a选项错误。
所以选C

Answer 2

31.c 文章的第一段提到了金属的一些特性，唯独没有提到答案c中的recyclability(可回收性)
32.d 有些软的金属可以混合在一起成为更加坚固的材料

a错误，文章中只是说混凝土内部的金属让建筑更牢固，并没有说混凝土必须和金属一起用于建筑。

b错误，文章中说alloy(合金), which is much stronger than either pure copper or pure aluminum，并不是比任何金属都要硬
c错误，文章中说Plastics are lighter and more corrosion-resistant(抗腐蚀的)，所以塑料的优势并不是cheaper(更便宜)
33. d 参见第三段
34. a 参见第三段
35. b 参见最后一段

Answer 3

看来也是今年的考生，35题老师给的答案A，以哪个为准