第一篇文章indus文明的衰落
第一部分讲一个古文明(印加)的衰落。 这个文明的衰落不是因为战争;也不是因为某一群人的定居，因为那群人是在衰落之后才到哪个地方去的。
第二部分讲overgrazing和deforestation 可能是原因。 因为人们到哪使用fireplace以及一些其他的自然资源。
第三部分另一个原因可能是降雨量。科学家发现，那个地方衰落的时候正好是降雨量减少的时候。还具体描述了在这段时期降雨量发生的变化，给出很多数据。
第四部分另一个原因是城市的繁荣跟周边rural地区食物的productivity有关。由于气候的原因，rural地区食物产量降低，大城市不能从周围地区获得食物就衰落了。同时，小地方没大城市衰落那么快，但最后变成了village。整个文明也就衰落了
解析：该篇文章为因果解释类文章，先描述现象印加文明的衰落然后解释不同的原因。典型的托福阅读cause-effect类文章。大家按照不同的理论整理文章结构即可。具体内容请大家参照如下文章。
Decline Of Harappan Civilization:
The Indus Valley Civilization dated from 3300 1300 BCE that was located in the north western region of the Indian Subcontinent. Flourishing around the Indus River basin, the civilization primarily centered along the Indus and the Punjabregion, extending into the Ghaggar-Hakra River valley and the Ganges-Yamuna Doab, encompassing most of what is now Pakistan, the western states of modern-day India, as well as extending into southeastern Afghanistan, and the eastern most part of Balochistan, Iran. The mature phase of this civilization is known as the Harappan Civilization, as the first of its cities to be unearthed was the one at Harappa, excavated in the1920s in what was at the time the Punjab province of British India (now in Pakistan).
The decline of Harappan culture is difficult to explain. During its late phase between 2000 and 1700 BC 'The Indus Valley Civilization as a distinct entity gradually ceased to exist'. Historians have different opinions regarding the causes of the decay and disappearance of the Harappan culture. Various causes have been ascribed for its weakening and then decay: Increase in rainfall, earthquake, decrease in fertility of soil, floods, Aryan invasion, disease etc. Mortimer Wheeler pointed out that the Harappan culture was destroyed by the Aryans. The Aryans were more skilled at warfare and were powerful than the Harappans. But there is very little evidence on this opinion.
.Another school of thought relates the demise of the Indus valley civilization to have been brought about by a major tectonic shift that caused continuous floods of this area. Research has proved that the decline of the glorious Harappan culture was due to a variety of factors, both manmade and natural.
In the beginning of the second millennium BC, there were great changes in the environmental conditions-the climate changed and large parts of the plains were flooded when tectonic changes threw up a dam in the lower Indus Valley. There were also other socio-economic factors that contributed to the decline. Agricultural production declined with the changes in the climate and the big cities could no longer sustain themselves. People from the major centers perhaps left for the smaller outposts and slowly riveted back to village life when they could no longer maintain the prerequisites of an urban existence. The causes of its decline are not certain. However, the physical existence of the civilization ended due to various factors: Economical changes led to the decline of land and agriculture, thereby enforcing the need to evacuate to other area might have been the reason for the disintegration of the Indus Valley. Shifts in the monsoon pattern and changes in temperature led to the areas more arid. Increase in Population, excessive deforestation, decline in agriculture etc. might have created economic problems leading to the gradual decay of the culture. The marked decline in the quality of building and town planning indicates that the authorities were losing control. The changes in the river flow pattern and correspondent widespread flooding would have disrupted the agricultural base.
