Maine will face a serious shortage of timber by the year 2000, the res

A lot of the confusion here is due to the OP's typo ("that" instead of "the" REALLY changes the sentence) and the posting of the incorrect answer. The full sentence will be, when correct:

Maine will face a serious shortage of timber by the year 2000, the result of a major infestation of spruce budworm, the coming to maturity of much of Maine's spruce and fir forests and a rapid expansion of the paper business.

This sounds fine and works well with parallelism, which is what this question is really testing.

the result of
- a major infestation of spruce budworm
- the coming to maturity of much of Maine's spruce and fir forests and
- a rapid expansion of the paper business

As I'm sure many people using this forum know, the word "much" is used with uncountable things, such as water and sugar. So why can we say "much of the forests"?

It's idiomatic to say "forests" this way, though quite old fashioned. "The forests of Maine are huge and full of owls." But we're not talking about countable numbers of individual forests, like, "there are 117 forests." We're using "forests" to describe something vast and uncountable. It's quite similar to the way we sometimes say "the waters of the Pacific" even though water isn't a countable noun.

This is old-fashioned, almost poetic phrasing in an old fashioned question, which is why you won't find it in any official guide from the last 25 years or so! Unless you're being held captive by an evil gang and your only way out is to get every old SC question right for the next two years, I wouldn't worry about this one.

Also, given that there isn't an option with "many" in it, you can just relax and worry about something else to help you pick your answer. Rather than obsessively proving that your answer is perfect as a diamond, prove that it's the best option of the five, or rather, that it's the least awful one of the five.

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