A.错,7.5g/cm^3=7.5g*1000/(1cm^3*1000)=7.5kg/dm^3 B错,6.5*10^3kg/m^3=6.5kg*10^3*0.001/(0.001m^3)=6.5kg/dm^3 C对 D过程不对,3.5g/cm^3=3.5*10^6g/((10^6cm^3)=3.5*10^3kg/m^3 选C