题目选项分析

Over the course of the eighteenth century, the average output of ironwork tripled as a result of several improvements in blowing machinery and because coal replaced charcoal as the fuel used in the smelting of iron ore.

Over the course of the eighteenth century a tripling in the average output of ironwork was due to the replacement of charcoal by coal for the fuel used in the smelting of iron ore, in addition to several improvements in blowing machinery.