Brown is not seen in a rainbow. A natural rainbow displays seven distinct colours — Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). These colours appear due to the dispersion of sunlight by water droplets in the atmosphere, but brown is not part of the visible spectrum formed by dispersion.

• Yellow, Green, Red: All are part of the seven colours of the rainbow.

Explanation: CNG stands for Compressed Natural Gas. It is an eco-friendly fuel made primarily of methane (CH₄) and is used as an alternative to petrol and diesel. CNG burns cleanly, producing fewer pollutants and helping reduce air pollution.

Explanation: Measles is a highly contagious viral disease caused by the measles virus (a member of the Paramyxovirus group). It spreads through droplets released when an infected person coughs or sneezes. Symptoms include fever, cough, rash, and watery eyes. Other diseases listed — scurvy, asthma, and diabetes — are non-communicable as they do not spread from person to person.

The normal hearing range of human beings is from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasonic sounds, while those above 20,000 Hz are known as ultrasonic sounds, both of which are inaudible to humans. With age or prolonged exposure to loud noise, the upper hearing limit tends to decrease, reducing sensitivity to high-frequency sounds.

Other options (for context)

• 30 to 30,000 Hz: Incorrect — exceeds the upper audible limit for humans.
• 50 to 50,000 Hz: Incorrect — animals like dogs and bats can hear this range, not humans.
• 40 to 40,000 Hz: Incorrect — the human ear cannot detect frequencies that high.

Explanation: Dehydration happens when the body loses excessive water through sweating, vomiting, diarrhea, or inadequate fluid intake. It disturbs the balance of electrolytes and reduces blood volume, leading to symptoms such as thirst, dry mouth, fatigue, and dizziness. To prevent dehydration, oral rehydration solutions (ORS) or adequate water intake are essential.

Explanation: Pathogenic parasites are those that invade the host’s body and cause disease. The presence of these harmful organisms stimulates the host’s immune system to produce antibodies that help neutralize or destroy the pathogens. Examples include Plasmodium (causes malaria) and Trypanosoma (causes sleeping sickness).

Explanation: In fused (molten) sodium chloride, electric current passes due to the presence of free ions. When solid NaCl is melted, it dissociates into Na⁺ and Cl⁻ ions, which can move freely and conduct electricity. In the solid state, these ions are fixed in the crystal lattice, so it does not conduct electricity.

Explanation: Opium is obtained from the latex present in the unripe capsule of the poppy plant (Papaver somniferum). The latex is collected by making incisions on the capsule, and after drying, it forms raw opium, which contains alkaloids like morphine and codeine.

Explanation: Tuberculosis (TB) is not a viral disease; it is caused by a bacterium called Mycobacterium tuberculosis. In contrast, Polio, Swine flu, and Hepatitis are viral diseases caused by different types of viruses.

Explanation: The acid present in the human stomach is hydrochloric acid (HCl). It helps in the digestion of food by breaking down proteins and activating the enzyme pepsin. HCl also kills harmful microorganisms present in the food, aiding in maintaining a healthy digestive system.