In industry, natural materials difficult to get are often replaced by plastics.
译文:在工业中,不易获得的天然材料常常被塑料代替。
Over the years, tools and technology themselves as a source of fundamental innovation have largely been ignored by historians and philosophers of science.
译文:工具和技术本身作为根本性创新的源泉,多年来在很大程度上被历史学家和科学的思想家忽视了。
Problems should be resolved in good time.
译文:问题应该及时加以解决。
The Apollo Ⅻ crew reported that their spaceship was being followed by two UFOs.
译文:阿波罗十二号的宇航员曾报告说,它们的宇宙飞船正遭到两个不明飞行物的跟踪。(用"遭到")
For separating iron from the impurities the iron ore must be melted.