原子中壳层电子吸收足够的能量脱离原子核的束缚变为自由电子的过程称为（）
A.基态
B.激发
C.电离
D.跃迁
E.特征光子
参考答案：C
—Maria is wanted on the telephone. Where is she?
—She _________ the library. You can find her there。
A．has been to
B．has gone to
C．has left
D．has been in
---By the way, how many of us can attend the meeting?
---The number of the students to attend the meeting _____ fifty.
A．is
B．has
C．are
D．have
