Sentence:  The big factories which are relatively near the centers of our cities - the rubber factories in Akron , Chrysler 's Detroit plants , U. S. Steel 's Pittsburgh works - often began on these sites <WSD>at</WSD> a time when that was the edge of the city , yet close to transport ( river ) , storage ( piers ) and power ( river ) . 
Meaning of the word marked with <WSD> tags:  simultaneously