They for two hours A. is workingB. workedC. have been workingD. had worked

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A. is working
B. worked
(答案→)C. have been working
D. had worked
解析：考查时态。现在完成进行时态表示的是从过去的动作开始一直持续到现在还要持续下去，故选C，句意：他一直工作了两个小时了。根据语境可知还要工作下去。