我有英语的阅读 有些不懂,请指教 这个是 toefl tpo16 第一篇

我有英语的阅读 有些不懂,请指教 这个是 toefl tpo16 第一篇Trade was the mainstay of the urban economy in the Middle East,as caravans negotiated the surrounding desert,restricted only by access to water and by mountain ranges.This has been so since ancient times,partly due to the geology of the area,which is mostly limestone and sandstone,with few deposits of metallic ore and other useful materials Ancient demands for obsidian (a black volcanic rock useful for making mirrors and tools) led to trade with Armenia to the north,while jade for cutting tools was brought from Turkistan,and the precious stone lapis lazuli was imported from Afghanistan.One can trace such expeditions back to ancient Sumeria,the earliest known Middle Eastern civilization.Records show merchant caravans and trading posts set up by the Sumerians in the surrounding mountains and deserts of Persia and Arabia,where they traded grain for raw materials,such as timber and stones,as well as for metals and gems我不懂得是 1 第一句话 Trade was the mainstay of the urban economy in the Middle East,as caravans negotiated the surrounding desert,restricted only by access to water and by mountain ranges.是由三个分句 组成,这三个分句的关系是什么?as 这里表示的是 原因还是 表示 时间?restricted 修饰的是前一句话还是 trade?access to water 还有就是 negotiate 是交涉的意思？还是谈判的意思？