C, Child laborers cost less money for business owners.
This is because children are paid less than adults, thus making it better for the capitalists in charge. During the Industrial Revolution (and even prior to it), factory owners used child labor because, quite simply, it was cheaper. Children worked for less than adults, and their parents were often happy to send them off to work, as the lower classes did not value education. Many men lost work when machines were introduced, as machines could do more work in less time, so if the children had the opportunity to work and earn money, they did.
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