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题目内容
（西安石油大学大学英语（2）作业题）
(单选题) Working hours in the United States ____ over the past 20 years. ( )(本题2.5分)
A、 were increasing
B、 have been increased
C、 have increased
D、 increased
A、 were increasing
B、 have been increased
C、 have increased
D、 increased
参考答案