我觉得c是独立主格吧。。。carcoal's being...的一长串是名词短语,和several improvements in...平行,所以是with+n+prep.+n的独立结构吧。。。但是a明显不平行,我也是刚刚做的这题,然后google到了此贴,而且我的正确答案给的是C,但我选的是b,但了前面大神们的解释知道了B错在不平行。。。
open to discuss...
有人知道b为啥错吗,我知道a tripling 不好, for the fuel used 的for也怪怪的。这里的due to用法正确吗?
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.