Answer:
D. When this was written, Shanghai’s economy relied largely on manual labor and support from their poor coolie classes
Step-by-step explanation:
The above text is referring to the Shanghai working class, which was basically made up of poor people. This is very evident when the narrator states that he stayed eight days in Shanghai and saw how people worked constantly and with poor working conditions, "they worked like animals, as the narrator says. In summary, the narrator shows that when that was written in writing, Shanghai's economy was largely dependent on manual labor and support from the poor coolie classes.
A historian can use this source to describe how economic processes and the working classes have evolved in China over time.