Glass （1)Since the Bronze Age, about 3000 B. C, glass has been used for making various kin

Glass

(1)Since the Bronze Age, about 3000 B. C, glass has been used for making various kinds of objects. It was first made from a mixture of silica, lime, and an alkali such as soda or potash, and these remained the basic ingredients of glass until the development of lead glass in the seventeenth century.

(2) When heated the mixture becomes soft and moldable and can be formed by various techniques into a vast array of shapes and sizes. The homogeneous mass thus formed by melting then cools to create glass, but in contrast to most materials formed in this way (metals, for instance), glass lacks the crystalline structure normally associated with solids, and instead retains the random molecular structure of a liquid. In effect, as molten glass cools, it progressively stiffens until rigid, but does so without setting up a network of interlocking crystals customarily associated with that process. This is why glass shatters so easily when dealt a blow.

(3) Another unusual feature of glass is the manner in which its viscosity changes as it turns from a cold substance into a hot, ductile liquid. Unlike metals that flow or "freeze" at specific temperatures, glass progressively softens as the temperature rises, going through varying moldable stages until it flows like a thick syrup. Each of these stages allows the glass to be manipulated into various forms, by different techniques, and if suddenly cooled the object retains the shape achieved at that point. Glass is thus open to a greater number of heat-forming techniques than most other materials.

According to the passage glass cools and becomes rigid differently from metals because ______.

A．it has an unusually low melting temperature

B．it does not set up a network of interlocking crystals

C．it has a random molecular structure of a liquid

D．it is made from a mixture of silica, lime, and soda