【单选题】Each unit is characterized by its ( ) state which changes in time.
【简答题】Density is the mass of a unit volume of material in a ( )state.
【简答题】Bulk density is the mass of a unit ( ) of powdery or granulated materials in a stacking state.
【简答题】( )is the mass of a unit volume of materials in a natural state.
【简答题】( ) is the mass of a unit volume of material in its
natural state.