Would it be d? ? and thanx! apex

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation:

Mary is conducting an experiment on how pollution affects plant growth. how can she ensure that her data are reliable?

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.