The correct answer is - A. Many unemployed farm workers could go to work in the country's factories.
Before the Industrial Revolution, most of the people were agricultural workers, but it was often happening that lot of them will have no work. With the start of the industrialization and the building of the production facilities, or rather the factories, all of those people that were unemployed in the agriculture got a chance to work in the factories. Also, the people that did had jobs as farm workers now had options, and that led to lot of them to switch jobs. Because of this, the labor force was abundant for the factories, which led to high productivity, everyone had a job if they wanted to, and the industry was able to constantly grow and develop.