工业上将相同物质的量的NaCl、CO2、NH3在30℃时配成溶液,此时溶液中无晶体。当降温至某温度时,开始析出晶体,此晶体是( )
A:NH4Cl
B:NH4HCO3
C:NaHCO3
D:NaCl
A pen is on the right of a ruler. The ruler is on the right of an eraser. That is to say, the ruler is ______ them.