factories应该是对的吧
因为后文,很后面又总结了一下
Hence, the characteristic new architectural forms of the late nineteenth and
twentieth centuries have been the factory, the multistory office building, the warehouse,the department store, the apartment house, the railway station, the large theater, and the gigantic sports stadium.
这些都相对应前文提到的没句话,而首先提到的就是factory,相应前文也首先提及
mechanized industrial production began to demand large, well-lighted interiors in which manufacturing could be carried on.况且manufacturing与factory也有关联啊
39.( During first )ten years of the Space Age, (which began) in 1957, (more than) 500 artificial satellites were rocketed (into orbit) around the Earth.