The development of the factory system in the early 1800s caused -

The factory system had a large impact on society. ... With the formation of large factories, people began to move to the cities. Cities grew larger and sometimes became overcrowded. This movement from a rural society to an urban society created a dramatic shift in the way people lived.

a is correct

hope this let me know if it is right

(d) rivers.

explanation:

civilizations developed around rivers because their waters provided places to hunt and fish. also, as the rivers flooded, the lands around them became fertile. this allowed them to support farming. this is especially true of the nile river, which flooded the same time each year.