The dissolved oxygen in a water sample is generally estimated by modified Winkler method. Accordingly, approximately 200 ml volume of dissolved-oxygen-fixed solution shall be titrated with

• A. Sodium thiosulphate reagent using soluble starch as an indicator
• B. Sodium thiosulphate reagent using ferroin as an indicator
• C. Ferrous ammonium sulphate reagent using soluble starch as an indicator
• D. Ferrous ammonium sulphate reagent using ferroin as an indicator