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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
Stalactites and stalagmites - two things
Hence we need are before common.
Therefore, D is out.
Commonly is wrong usage in option E.
Therefore E is incorrect.
We are now left with A, B and C
In Option B, "That" is trying to show that the minerals were formed over time, but its not the minerals but the stalactites and stalagmites.
In GMAT
Verb form > Adjective Form > Noun Form
This means that verb form has the highest preference and then adjective and then the least noun.
In C, "the dripping of water" is the noun form
Whereas, In A, "dripping water" is adjective form.
Hence C is ruled out.
Answer is A.