公元2世纪，我国古代炼丹家魏伯阳著有《周易参同契》，这是世界上现存最早的一部炼丹专著。书中描写道：“金入于猛火，色不夺精光。”这句话是指黄金的性质在强热条件下(   )

教室门框的高度最接近于（　　）

    We can see different signs in public places．Some____tell us what we should do．This sign is____．There is a picture of an animal in the____．It tells us to look out for animals．Some signs tell us what we should not do．We should not smoke in the forest．It____start a fire．That's why we see a "____" sign in the forest．Some signs tell us where to go．The bus and underground signs tell us where to find the stations．