Read the summary about factory work during the Industrial Revolution. Factory work was not easy. First of all, it involved long hours. Typical factory workers put in twelve-hour days, six days a week, and there were few holidays. Moreover, most workers had to do the same simple tasks over and over each day. At the same time, they had to work fast to keep pace with the machines they tended. Finally, factory work could be dangerous. Fingers, hair, and clothing often got caught in the many gears and belts of machines, making accidents common. Also, textile mills were filled with cotton dust, which damaged the lungs of the workers who breathed it in.

The underlined sentence states
A: a central trend.
B: some key details.
C: the writer’s opinion.
D: a summary statement.