Pick up the correct statement from the following :

• Hydrogen sulphide gas in excess, may cause corrosion of concrete sewers
• 4 ppm of Dissolved Oxygen (D.O.) is ensured before discharging the treated sewage in river
• Solubility of oxygen in sewage is 95% of that of distilled water
• Dissolved oxygen content of sewage, may be determined by Winkler's method
• All the above.