What is true of diffusion?
a- diffusion is only important for animal cells
b- only water can diffuse across a cell membrane
c- diffusion requires energy input from the cell
d- molecules move from areas of high concentration to areas of low concentration

The content of manganese (mn) in steel was determined spectrophotometrically and with the use of the standard addition method. an unknown sample of mn from a digested steel sample gave an absorbance of 0.185 when analyzed spectrophotometrically. when 5.00 ml of solution containing 95.5 ppm mn was added to 50.0 ml of the unknown steel solution (digested sample), the absorbance was 0.248. calculate the concentration, in parts-per-million (ppm), of mn in the digested steel sample solution.