I believe that the answer is C. In the decade following the Civil War, tall buildings had just begun to emerge; and, although the skylines of the world’s great cities were still dominated by church spires, there was increasing activity in the development of elevator apparatus adapted to the transportation of people as well as of merchandise.
Step-by-step explanation:
This is because, the whole text is about the elevator and its development throughout the years. This paragraph explains the development of the elevator, its possible purpose, and the time frame in which it took place. This sentence seems to sum up the text better than the other options.
The correct answer for the question that is being presented above is this one: "d. keystone." The part of an arch that projects slightly from the wall and the point where the arch springs from is called the keystone.
