The reagent that can be used to distinguish acetophenone and benzophenone is ______.

When propionic acid is treated with aqueous sodium bicarbonate, ${\text{CO}}_{\text{2}}$ is liberated. The ${\text{C of CO}}_{\text{2}}$ comes from the _________. 

An organic compound, ${\mathrm{C}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{6}}\mathrm{O}$ does not give a precipitate with $2,4$-dinitrophenylhydrazine reagent and does not react with metallic sodium. It could be_______.