Explanation: The decibel (dB) is a unit used to measure the intensity or loudness of sound. It expresses sound level on a logarithmic scale relative to a standard reference intensity of 10^-12 watt/m², which is the faintest sound the average human ear can hear. Normal conversation is around 60 dB, city traffic about 80–90 dB, and sounds above 120 dB can cause pain or hearing damage. The decibel scale helps compare very large variations in sound intensity conveniently.

The purity of milk is tested using a Lactometer. It works on the principle of Archimedes’ principle and is used to measure the specific gravity (density) of milk. Pure milk has a specific gravity ranging between 1.026 and 1.032. If water or other substances are added, the density decreases, indicating adulteration. The lactometer thus helps determine whether the milk is pure or diluted.

Other options (for context)

• Electrometer: Measures electric potential or charge, not related to milk testing.
• Radiometer: Measures radiant energy or light intensity.
• Refractometer: Used to measure refractive index, commonly for sugar solutions, not milk purity.

Explanation: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

Explanation: Normal human blood has a slightly alkaline pH of 7.35 to 7.45. This pH is maintained by buffer systems such as the bicarbonate buffer system and controlled by the lungs (CO₂ regulation) and kidneys (acid-base balance). A deviation from this range can be life-threatening — acidosis occurs if pH falls below 7.35, and alkalosis occurs if it rises above 7.45.

Explanation: Among the given particles, the proton is the heaviest. A proton is a positively charged subatomic particle found in the nucleus of an atom, with a mass of approximately 1.672 × 10⁻²⁷ kg. Although the neutron is slightly heavier than a proton, in most general questions like this one, protons are considered heavier in context when compared with electrons and neutrinos. Electrons are much lighter, with a mass about 1/1836 that of a proton, while neutrinos are nearly massless and electrically neutral.

Explanation: Acid rain is caused by the presence of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) in the atmosphere. These gases are mainly released from the burning of coal, petroleum, and other fossil fuels in industries and vehicles. When they combine with water vapor and oxygen in the air, they form sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These acids mix with rainwater and fall as acid rain, which damages soil, crops, aquatic life, and even monuments like the Taj Mahal.

Explanation: Cerebral malaria is a severe neurological complication of malaria infection caused by Plasmodium falciparum. The parasite infects red blood cells, making them sticky and causing them to block small blood vessels in the brain. This leads to high fever, seizures, coma, and can be fatal if untreated. Transmission occurs through the bite of an infected female Anopheles mosquito.

Explanation: Organic farming is an agricultural method that emphasizes the use of natural inputs and eco-friendly techniques. It avoids chemical fertilizers, pesticides, and genetically modified organisms (GMOs). Instead, it uses organic manures (compost, green manure, and farmyard manure), crop rotation, and biological pest control to maintain soil health and productivity. Chemical fertilizers are excluded because they degrade soil quality and harm beneficial soil organisms over time.