下列试剂中,能用于检验酒精中是否含有水的是[ ]
A．CuSO4·5H2O
B．无水硫酸铜
C．浓硫酸
D．金属钠
答案：B
“若a∉M或a∉P，则a∉（M∩P）”的逆否命题是______．
There are about ____________ students in our school.
A．four thousand
B．four thousands of
C．four thousand of
D．thousand of
