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To tell you the truth, it.s very hard for us to help them get rid of Internet addiction. But we _________ on this problem trying to improve the situation.

（4.0分）4.0 分
A、
had worked

B、
are working

C、
had been working

D、
worked

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出自：郑州航空工业管理学院-土木工程-公共英语（3）

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