GMAT Sentence Correction (SC) | Over the course of the eighteenth century, the average output of ironw

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

A normal construction would be - as a result of several improvements ..... and replacement of coal
In this sentence, by introducing "because" the 2nd clause (coal replaced charcoal..) is not made parallel (this seems to acceptable)

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