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Around 1850 there were about 800 farms in Otsego County. By the 1950's the number of farms had dropped to around 400. By 1988 there were only 81 farms in operation. Therefore, the amount of land in the county that is devoted to farming has dropped by about 90 percent in the past 140 years.

Okay! So I know, the author is neither talking about how or which crops were grown, so all answers, including "Crops" are out of scope this negates option E. Lets check other answers choices.

A) counts the number of farms only approximately- This is not an issue as the % given is also approximately, this won't create much difference.- OUT
(B) fails to say whether the use now being made of the land previously devoted to farming is income-producing- Income producing? Sorry? This is absolutely and completely out of scope-OUT
(C) ignores the possibility that the average size of farms has changed
(D) gives data for only 3 of the past 140 years-Doesn't matter, why? because the stem only considers the 2 years 1850 and 1988 to know the change and all the other years can be irrelevant to calculate % change- OUT

So, even without checking option C by removing other option, I know C is the answer. But to explain, lets say the total area under farmland was 800sq.mts which was owned by 800 diff people. Now, it is possible that the farms were the bought by a wealthy person and were merged together and thus, the number of farms decreased, but the area of the farms did not. This is just a hypothetical example. But C is the only viable option.

Let me know if any further queries!

Hope this helps!