数据是记录下来可鉴别的（）。
参考答案：符号和数字
含有3.01×1023个氧原子的H2SO4的物质的量是
A．1 mol
B．0.5 mol
C．0.25 mol
D．0.125mol
Several weeks had gone by____________I realized the painting was missing.
A． as
B． before
C． since
D． when
