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.
Explanation:
The above sentence most aptly backs the advancement of key idea of 'advent of elevator system in United States and its spread.' The development of <u>'elevator apparatus adapted to the transportation of people as well as of merchandise' assisted in promoting the hotel operators to 'elevate their patrons for one or two floor above</u>' <u>for commercial gains that led the foundation of early elevator industry</u> that eventually kept on advancing with time. It reflects that how the apparent advantages of elevator system have played a significant role in emergence of tall buildings as well. Thus, <u>option C</u> is the correct answer.
