【单选题】— Has Sam finished his homework today? — I have no idea. He ________ it this morning.
【单选题】在讨论铅酸蓄电池结构时,甲说 12V 铅酸蓄电池由 6 个单格电池并联组成, 乙说,12V 铅酸蓄电池由 6 个单格电池串联组成,你以为( )。
【简答题】— Has Sam finished his homework today, Anne? — I have no idea. But he ________ it this morning. [ ] A .did B. had done C. was doing D. had done