原子序数为12的镁（Mg），核外电子分布于（）个电子层上.
A.3
B.4
C.2
D.1
参考答案：A
He didn't believe it was true until he________her offer of a job.[ ]
A．swept up
B．sped up
C．picked up
D．took up
阅读下面程序 import jav
A．io．*； pub
