单项选择题

A need for beauty, lightness, corrosion resistance, or other specific properties must be present before plastic can be considered as competitors of brick, window glass, cement, cast iron, or steel, since volumetric prices are so low for the last substance and for wood. It is not particularly unfortunate that plastics do not appear economical for every use. There is no reason why industry should want to replace wood, brick, concrete, and metals when the latter are adequate and inexpensive. Too much has been written about the coming "Plastics and Light Metal Age," which is prophesized as the successor to the Stone Age, the Bronze Age, and the Iron Age. In the historical sequence of these earlier periods, there is logic in the quantitative sense; one age gave way to another when the use of a new material exceeded in quantity that of its predecessor. For the plastics and light metals, however, a different picture presents itself; less than 3,000,000 tons of all these materials are being produced annually, while steel production exceeded 90,000,000 tons last year and will probably not recede to less than 60,000,000 tons for many years. This is still the Iron Age or rather the Steel Age.
The writer considers that the Bronze Age succeeded the Stone Age because