Which answer would score higher? Not sure where to start? Explain this Answer A Because 0.40 mol O₂ is less than 0.50 mol Mg, oxygen limits the reaction and would form 0.80 mol MgO; magnesium remains in excess. Answer B Because 0.50 mol Mg needs only 0.25 mol O₂, magnesium limits the reaction and forms 0.50 mol MgO; 0.15 mol O₂ remains.