Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used to correctly mark the boundary between one sentence ("Before...area") and another ("After...area"). The phrase "After construction" modifies the next sentence to indicate that the temperature was cooler after construction.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence that fuses the two sentences ("Before...area" and "after...area") without punctuation and/or a conjunction.
Choice B
Choice B is incorrect because it results in a run-on sentence that connects the two sentences ("Before...area" and "after...area") without punctuation and/or a conjunction. Additionally, no punctuation is needed between "after" and "construction."
Choice D
Choice D is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between the two sentences ("Before...area" and "after...area").