The workers were made twelve hours a day.A. workB. to workC. workedD. working

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A. work
(答案→)B. to work
C. worked
D. working
解析：这些工人每天被迫工作十二个小时。make sb. do sth.让某人做某事，其中的to是省略掉的， 但用于被动的时候需要将to还原，也就是： … be made to do，故选B。