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i _______for three hours when i suddenly became sick.

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i _______for three hours when i suddenly became sick.
A.had been working B.would work C.have worked D.am working
would work是本应该工作.have worked是过去完成时,与正好突然生病不对.am working就不解释了.had been working是“过去三个小时一直在工作”.