Why Did the Cities of the Late 19th Century Begin

Why did the cities of the late 19th century begin to grow vertically?

A) Technological innovations made the skyscraper possible, and it became the housing format of choice.
B) Congested and densely packed, many cities could only grow up.
C) The new economy required space for the armies of blue-collar workers.
D) Environmental restrictions limited the possibilities for cities to develop horizontally.