【单选题】

The following questions present a sentence, part of which or all of which is underlineD、Beneath the sentence, you will find five ways of phrasing the underlined part. The first of these repeats the original; the other four are different. If you think the original is best, choose the first answer; otherwise choose one of the others.

These questions test correctness and effectiveness of expression. In choosing your answer, follow the requirements of standard writtenEnglish; that is, pay attention to grammar, choice of words, and sentence construction.Choose the answer that produces the most effective sentence; this answer should be clear and exact, without awkwardness, ambiguity, redundancy, or grammatical error.
At ground level, nitrous oxides are bad enough, but when up high, atmospherically they bond with free ions to create dangerous smog particles.
A、but when up high, atmospherically they
B、however, it is in the upper atmosphere in which it may
C、but in the upper atmosphere, they
D、however, but once in the upper atmosphere it is known to
E、as in the upper atmosphere they