Which of the following sentences is punctuated correctly? A. On a farm, the smell of manure is so, strong that the air—once clean and fresh, flees from the odor. B. On a farm, the smell of manure is so strong that the air—once clean and fresh—flees from the odor. C. On a farm, the smell of manure is, so strong that the air, once clean and fresh flees from the odor. D. On a farm, the smell of manure is so strong that the air (once clean and fresh flees from the odor).

B. On a farm, the smell of manure is so strong that the air—once clean and fresh—flees from the odor.

Explanation:

Since more information about the word 'air' is provided, this description needs to be separated from the rest of the sentence. This is sometimes achieved by the use of commas, while in other cases (as in option B) dashes are used. The correct punctuation is, therefore, found in option B. In option A, the punctuation is not correct, as there is an unnecessary comma after the word 'so'. In option C, there is also an extra comma after the verb 'is'. In option D, the parenthesis are not used correctly.