重点讲解：
1. perch on
栖息；
eg. The bird took its perch on a tree branch.
鸟儿栖息在树枝上。
eg. An eagle found a perch on the rock.
一只老鹰在岩石高处落脚。
eg. Gilded parrots perch on candleholders, their tiny tilted heads reflected in mirrors.
镀金的鹦鹉栖息在烛台上，它们斜侧着的小脑袋映照在镜中。
2. go back
追溯到；回溯到；
eg. Our association with him goes back four years.
我们同他的合作是从4年前开始的。
eg. We must go back to the Middle Ages to discover the origins of English plays.
我们必须追溯到中世纪才能找到英国戏剧的起源。
eg. To trace the origins of the Irish problem, we have to go back over three hundred years.
要探讨爱尔兰问题的起因就要追溯到三百多年以前。