What increased the number of deaths during the triangle shirtwaist fire

The owners had locked doors to control workers' breaks

Locked doors is the correct answer.

Explanation:

The triangle shirtwaist company factory fire in  New York city killed more than 140 workers. It was one of the most infamous incidents because  the death of workers could have been prevented. Neglected safety features and locked doors in the building were two causes of deaths. This incident shed light on the working conditions in factories and led to the development of many laws for the safety of workers.

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