—Has Tom finished his task yet?—I’m not sure. But he on it last weekA. was workingB. has

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(答案→)A. was working
B. has been working
C. worked
D. had been working
解析：考查时态。句意：——Tom完成了他的任务吗？——我不确定。但是他上周一直致力于这个任务。根据语境，说明与过去有关，且表示过去一段时间（last week）内一直在做某事，故使用过去进行时。