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 written English; 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.
Stalactites and stalagmites, accretions of minerals formed over time by dripping water, are common features in the region’s corners.（）
A. formed over time by dripping water, are common
B. that are formed by water dripping over time, are common
C. formed through time by the dripping of water, are common
D. having formed over time through dripping water, is a
E. which are formed over time by the dripping of water, are commonly