sat-dog

Choice D is the best answer. The word “falling” occurs in the middle of a clause and isn’t part of a supplement, so we don’t need any punctuation after it. We can see this more clearly if we simplify the rest of the sentence: “The reason seeds appear to float while falling is that their plumes enhance drag.” .

Choice A is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. The word “falling” occurs in the middle of a clause and isn’t part of a supplement, so we don’t need any punctuation after it. We can see this more clearly if we simplify the sentence: “The reason seeds appear to float while falling is that their plumes enhance drag.” . Choice B is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. The word “falling” occurs in the middle of a clause and isn’t part of a supplement, so we don’t need any punctuation after it. We can see this more clearly if we simplify the sentence: “The reason seeds appear to float while falling is that their plumes enhance drag.” . Choice C is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. The word “falling” occurs in the middle of a clause and isn’t part of a supplement, so we don’t need any punctuation after it. We can see this more clearly if we simplify the sentence: “The reason seeds appear to float while falling is that their plumes enhance drag.” .