A选项用rainfall，B选项用melting snow定位到：
Rainfall was limited, and what water there was rushed across the plain in the annual flood of melted snow.
AB选项和原文呼应，所以不选。
C选项用silt定位到：
Once this was done and the silt carried down by the rivers was planted, the rewards were rich: four to five times what rain-fed earth would produce.
这里说了带来的回报是丰厚的。所以C选项的damaged crops很原文说反，选C。
D选项的stone, metal对应到：
There were few natural resources, no timber, stone, or metals.
D正确，不选。
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