For thousands of years, people thought of glass as something beautiful to look at. Only re

cently 【C1】______ come to 【C2】______ something to look through. Stores 【C3】______ their goods in large glass windows. Glass bottles and jars 【C4】______ food and drink allow us 【C5】______ the contents. Glass 【C6】______ spectacles (眼镜), microscopes, telescopes, and 【C7】______ very useful and necessary objects. 【C8】______ are used by people who cannot see 【C9】______ or by people who want to protect their eyes 【C10】______ bright light. Microscopes make tiny things larger 【C11】______ we can examine them. Telescopes 【C12】______ objects that are far away appear 【C13】______ closer to us.

【C14】______ in recent years plastics have replaced glass 【C15】______ conditions where glass might be 【C16】______ broken there are new uses 【C17】______ for glass that were never imagined in the 【C18】______ . Perhaps the greatest 【C19】______ . of glass is that its constituent (形成的) parts are inexpensive and can be found 【C20】______ over the world.

【C1】_____

A．they

B．do they

C．they have

D．have they