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