Look at the four squares,A ,B ,C , and D, which indicate where the following sentence coul

d be added to the passage. Where would the sentence best fit？

Sophisticated plumbing was needed to service bathrooms and also to heat buildings with either steam or hot water.

A Skyscrapers differed from previous tall structures with their use of technical innovations such as cast iron and the elevator. The development of cast iron technology, in which molten iron is poured into a mold, made modem plumbing possible.B Cast iron pipes, fittings, and valves could deliver pressurized water to the many floors of tall buildings and drain wastewater out. The invention of the mechanical elevator made it possible to construct even taller buildings.C Before the elevator, office buildings were rarely more than four or five stories high. In 1857, the first passenger elevator equipped with safety brakes prevented the elevator from falling to the basement when a cable broke. The elevator made the upper floors as rentable as the first floor, liberating architecture from dependence on stairways and human muscle.D

A．Square Ａ.

B．Square Ｂ.

C．Square Ｃ.

D．Square Ｄ.