Explanation

Creating a filter: the prompt is quite a mouthful. Where are the commas? We might suspect that there is a comma-related defect, but glancing over the answer choices, we can notice that there are no commas. Sentence Correction questions rarely involve comma defects, with the exception of comma splices, although commas are important to identify modifiers and otherwise parse sentences. We can move on and look for objective defects.

Finding objective defects: we have a defect in (B); the pronoun "it" means the sentence discusses what "this year's mean temperature" was "50 years ago," which makes no sense and is contrary to the intended meaning. Choice (C) has the same problem. They all do! Except (A). A question that otherwise might have been quite confusing ended up crumbling under one error. The correct answer is (A).