Over the course of the eighteenth century, the average output of ironw

"as a result of xxxx" and "because yyyy" both modify/describe/explain the action of the preceding clause. There's no choice that creates this type of construction with better parallelism, so, go with this one.






To resolve this split, use the usual combination of context and common sense to determine which words SHOULD have which relevant functions/relationships.

Here, you should concentrate on the modifier "over the course of the 18th century"—a modifier that's common to the two choices, but whose referent differs between them.

Ask yourself:
WHAT ••HAPPENED•• "over the course of the 18th century"? (...and please note that this modifier has to describe something that legitimately could have HAPPENED OVER A TIME PERIOD!)

A/ "Output tripled"
This is EXACTLY what happened over the 18th century. (In other words, the output in 1799 was three times the output in 1700.)

B/ "The tripling was due to..."
This is NOT an event that took place over a historical time period. Instead, this is a historical assertion that is factually either true or false—and its status as true or false is timeframe-independent (i.e., if this statement is historically accurate, then it will remain true forever).

A therefore makes sense while B is nonsense.