OG详解-OG3 文法 Q4

Choice D is the best answer. The convention being tested is punctuation between a verb and a preposition. When, as in this case, a verb (“is added”) is immediately followed by a preposition (“whenever”), no punctuation is needed. 
Choice A is incorrect because no punctuation is needed between the verb andthe preposition. Choice B is incorrect because no punctuation is needed between the verb and the preposition. Choice C is incorrect because no punctuation is needed between the verb and the preposition.