— Has Tom finished his homework?
— Sorry, I don't know. He ________ it this morning when I saw him.
A：did
B：does
C：has done
D：was doing
①面带微笑 ②真诚夸赞 ③寻找不同之处 ④忘记对方的名字
研究电磁铁磁性的强弱与线圈的匝数的关系时，要改变（ ）。
文学常识和名著阅读。
