（2012）钴的价层电子构型是3d74s2，钴原子外层轨道中未成对电子数为：（）
A.1
B.2
C.3
D.4
参考答案：C
The secretary worked all night long, ______ a long speech for the president.
[ ]
A. to prepare
B. preparing
C. prepared
D. was preparing
二氧化碳燃烧产物，红外尾气分析仪按容积百分比计量排放水平。
