If the engineering feats of that early decade remains impressive - boring a mile-long tunnel through a solid mountain, and turning the eastward flow of two rivers westward - the arduous conditions in which the engineers worked are no less remarkable.

A. If the engineering feats of that early decade remains impressive - boring
B. If the engineering feats of that early decade remains impressive - such as the boring of
C. Although the engineering feats of that early decade remains impressive - boring
D. If the feats of engineering in that early decade remains impressive - boring
E. If the engineering feats of that early decade remain impressive - boring