What do you think would have been the hardest part of using the cameras and photographic processes that existed before the 20th century

One of the hardest parts of using the photographic processes that existed before the 20th century was the fact that these required a long exposure time. Therefore, it was impossible to take pictures of fast-moving objects, such as cars or animals. When photographing people, they needed to remain still for a very long time, which means that their poses were often lifeless or overly serious. Other important disadvantages were the cost and the fact that making copies was difficult or impossible, depending on the original method.

i'm not too sure - if you could attach the picture

answer; i believe the correct answer is in new york,in subway stations,public murals and night clubs;