下列生产性毒物中，以蒸汽的形式散发于空气中的是( )。
A．铬酸
B．水银
C．铜
D．农药
参考答案：B
Swimming is my favorite sport. There is _____ like swimming as a means of keeping fit. [ ]
A. something
B. anything
C. nothing
D. everything
试述烟草杂交种选育程序
