Explanation: Chlorofluorocarbons (CFCs) are the gases responsible for the depletion of the ozone layer. When released into the atmosphere, CFCs break down under ultraviolet (UV) radiation, releasing chlorine atoms that destroy ozone molecules (O₃), leading to thinning of the ozone layer and increased UV radiation reaching the Earth.

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.

When iron rusts, its weight increases. This is because rusting is a chemical reaction between iron, oxygen, and moisture from the air, forming hydrated iron(III) oxide (Fe₂O₃·xH₂O). During this process, oxygen atoms from the air combine with the iron, adding extra mass to the metal surface in the form of rust. However, while the weight increases, the structural strength of the iron decreases due to corrosion.

Other options (for context)

• Decreases: Incorrect — mass increases due to the addition of oxygen.
• Remains the same: Incorrect — oxygen from air chemically adds to iron, increasing total mass.
• First increases and then decreases: Not applicable — in normal rusting, weight continues to increase until the surface flakes off.

The Red Data Book contains information about endangered and threatened species of plants and animals. It is published by the International Union for Conservation of Nature (IUCN). The book categorizes species based on their risk of extinction, such as Critically Endangered, Endangered, Vulnerable, and Extinct. It serves as a global reference for monitoring biodiversity conservation and guiding environmental protection policies.

Other options (for context)

• All plant species: Incorrect — it includes only those plants that are endangered or threatened.
• All animal species: Incorrect — it covers only the endangered and vulnerable ones, not all animals.
• All extinct species: Incorrect — it includes species at risk of extinction, not only those already extinct.

Sound waves from a loudspeaker are produced by vibrations of its diaphragm. When an electric current passes through the speaker coil, it causes the diaphragm to move back and forth, creating compressions and rarefactions in the air. These pressure variations travel as sound waves to our ears.

• Frequency: Determines pitch but does not create sound.
• Pitch: A property of sound, not its cause.
• Amplitude: Determines loudness but not the origin of sound.

Explanation: The liver is the largest gland in the human body. It is located in the upper right part of the abdomen, just below the diaphragm. The liver performs multiple vital functions, including the secretion of bile (which helps in digestion of fats), detoxification of harmful substances, storage of glycogen (energy reserve), and the synthesis of proteins and cholesterol. It also plays a crucial role in regulating blood sugar levels and metabolism. The average adult human liver weighs about 1.2 to 1.5 kilograms.

Swine Flu is caused by the H₁N₁ virus, a subtype of the Influenza A virus. It primarily affects the respiratory system and is transmitted from person to person through droplets from coughs or sneezes. Symptoms include fever, cough, sore throat, body aches, and fatigue. The H₁N₁ pandemic in 2009 was declared a global health emergency by the World Health Organization (WHO).

Other options (for context)

• Dengue: Caused by the Dengue virus, transmitted by Aedes mosquitoes.
• AIDS: Caused by the HIV (Human Immunodeficiency Virus), not H₁N₁.
• Bird flu: Caused by the H5N1 virus, not H₁N₁.

Explanation: Cobalt-60 (Co-60) is a radioactive isotope of cobalt that emits high-energy gamma rays during its decay process. It decays into Nickel-60 (Ni-60) by beta decay, releasing two strong gamma photons of energies 1.17 MeV and 1.33 MeV. Due to these intense gamma emissions, Co-60 is widely used in radiation therapy (teletherapy) for cancer treatment, as well as in industrial radiography and sterilization of medical equipment. Gamma rays have high penetration power, allowing them to destroy cancerous cells deep inside tissues.

Explanation: The Curie point (or Curie temperature) is the temperature above which a ferromagnetic material loses its magnetic properties and becomes paramagnetic. At this point, thermal energy overcomes the magnetic alignment of atoms. For example, the Curie temperature of iron is about 770°C.