David a customer when the fire startedA. servedB. was serving C. had servedD. would serv

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A. served
(答案→)B. was serving
C. had served
D. would serve
解析：句意：当大火开始时正在服务一个客户。从从句的谓语动词可知与过去有关，四个选项均为过去时态。这里表示一个动作发生的时候，另一个动作正在进行。故用过去进行时。