. Tom as well as his classmates a song when the teacher came into the classroom.A. are si

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A. are singing
B. were singing
C. is singing
(答案→)D. was singing
解析：考查主谓一致。as well as连接两个主语时，谓语动词取决于前者，句意：当老师进入教室的时候，不仅汤姆的同学而且汤姆当时正在唱歌。根据语境表示过去某一时刻进行的动作应用过去进行时态，所以选D。