Explanation: RADAR (Radio Detection and Ranging) works by using radio waves to detect and determine the distance, speed, and position of objects such as aircraft, ships, and vehicles. The radar system transmits radio waves, which travel through the air, hit an object, and get reflected back to the radar receiver. By measuring the time delay between transmission and reception, the radar calculates the object's distance and motion. Radio waves are ideal because they can travel long distances, penetrate clouds, and are unaffected by weather conditions.

Explanation: The disease AIDS (Acquired Immuno Deficiency Syndrome) is caused by the HIV (Human Immunodeficiency Virus). HIV attacks the body’s immune system, specifically the CD4 or T-helper cells, weakening the body’s ability to fight infections and diseases. It spreads through unprotected sexual contact, sharing of infected needles, transfusion of contaminated blood, or from an infected mother to her child during birth or breastfeeding. There is no permanent cure, but antiretroviral therapy (ART) helps control the virus and prolong life.

Explanation: Nitrogen (N₂) constitutes the major portion of the Earth's atmosphere by volume — about 78%. It is a colorless, odorless, and inert gas that plays a vital role in maintaining atmospheric stability and supporting life through the nitrogen cycle. Oxygen makes up about 21% of the atmosphere and is essential for respiration and combustion. Other gases like argon (~0.93%) and carbon dioxide (~0.04%) are present in much smaller amounts.

Nitrogen dioxide (NO₂) is a major cause of acid rain. When released into the atmosphere from vehicle exhausts and industrial emissions, it reacts with water vapor to form nitric acid (HNO₃). Along with sulfur dioxide forming sulfuric acid, it contributes to acid rain, which damages vegetation, aquatic life, and buildings.

• Carbon dioxide: Causes global warming, not acid rain.
• Carbon monoxide: A toxic gas but not acidic.
• Oxygen: Essential for respiration, not a pollutant.