Прочитайте текст и выполните задания Plastic Materials 1. Plastic materials are not found in nature....: ответ на тест 30187 - Английский язык

Прочитайте текст и выполните задания
Plastic Materials
1. Plastic materials are not found in nature. Plastic is formed by extrusion or injection molding under very high pressure. It can be molded into any desired shape. Organic plastics are divided into two general groups: thermosetting and thermoplastic. The thermosetting group becomes rigid through a chemical change that occurs when heat is applied. These plastics cannot be remolded. The thermoplastic group remains soft at high temperatures and must be cooled before becoming rigid. This group is not used generally as a structural material.
2. Plastics are rapidly becoming important construction materials because of their great variety, strength, durability and lightness. Plastics are light. The benefits of light weight coupled with good strength and absence of corrosion offer tremendous potential as alternatives to traditional building materials. A given volume of polythene weights less than one-eighth of an equal volume of iron and less than half of the same volume of aluminum.
3. Plastics are used in the industry and in the household: from rockets and electronics to toys and house ware. Plastic products offer many advantages over the materials they replace, such as ease of handling, lower maintenance costs and rapidity of assembly. The insulation and dielectric properties of plastics led to their early use in the electrical engineering industry, which was followed by special application in mechanical engineering.
4. Using of plastics as materials for a construction in the form of sheets, rods or tubes is substituting the conventional metals. Plastics offer a lot of properties for the designs. Plastics have now been developed to such an extent that they can be applied to almost every branch of building, from the laying of foundations to the final coat of paint.
Определите основную идею текста.