The highest tides, known as spring tides, occur during the time of the new moon and the full moon. During these phases, the Sun, Moon, and Earth are aligned in a straight line, causing their combined gravitational forces to produce higher high tides and lower low tides. This alignment amplifies the tidal effect, resulting in the strongest tidal ranges.

Other options (for context)

• Solar eclipse: Can coincide with a new moon but is not directly responsible for higher tides.
• Lunar eclipse: Occurs during a full moon but does not always produce exceptionally high tides.
• Volcanoes: Have no relation to tidal movements; tides are caused by gravitational forces, not geological activity.

William Harvey, an English physician, discovered the process of blood circulation in 1628. He demonstrated that the heart acts as a pump to circulate blood continuously throughout the body via a closed system of arteries and veins. His work, published in "De Motu Cordis" (On the Motion of the Heart and Blood), laid the foundation for modern cardiovascular physiology.

Other options (for context)

• Mary Anderson: Invented the windshield wiper, not related to medicine.
• Virginia Apgar: Developed the Apgar score for assessing newborn health.
• Robert Feulgen: Discovered a staining technique for DNA, not blood circulation.

Sound becomes hazardous and causes noise pollution when its intensity exceeds 80 decibels (dB). Prolonged exposure to such high noise levels can lead to hearing loss, stress, sleep disturbances, and other health problems. Common sources include heavy traffic, loudspeakers, machinery, and construction activities. Sounds above 120 dB can cause immediate ear damage or pain.

Other options (for context)

• Above 50 dB: Considered moderate — typical of normal conversation or office noise.
• Above 100 dB: Very loud and potentially painful, but 80 dB is the threshold for long-term hazard.
• Above 130 dB: Extremely loud, causes immediate physical pain and ear damage.

Explanation: Red light is used as a stop signal because it has the longest wavelength among visible colors, allowing it to scatter the least and remain visible from a greater distance, even in fog, rain, or dust. This ensures better visibility and safety on roads.

Explanation: A heavy stone appears lighter when immersed in water because of the upward pressure or buoyant force exerted by the water. This phenomenon is explained by Archimedes’ Principle, which states that a body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced by it. The greater the volume of the object submerged, the greater the buoyant force acting on it. Hence, the stone seems lighter in water, even though its actual mass remains the same.

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.

Explanation: Photosynthesis is the process by which green plants, algae, and some bacteria use sunlight to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂). This process takes place in the chloroplasts of plant cells, which contain the green pigment chlorophyll that captures solar energy.

Chemical Equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Thus, plants are known as autotrophs because they produce their own food using light energy — a process vital for sustaining all life on Earth.