Explanation:

1 and 2 only

Water pollution is the addition/presence of undesirable substances to/in water such as organic, inorganic, biological, radiological, heat, which degrades the quality of water so that it becomes unfit for use’.

• The higher amounts of waste increase the rates of decomposition and O2 consumption thereby decreases the DO content of water.Thus decrease in the level of dissolved oxygen is an indicator of water pollution.
•  Presence of organic and inorganic wastes in water decreases the dissolved      oxygen content of the water.

Discharge of hot water may increase the temperature of the receiving water by 10 to 15 °C above the ambient water temperature. This is thermal pollution.

• Increase in water temperature decreases dissolved oxygen in the water.