Explanation: Heavy water is chemically known as Deuterium oxide (D₂O). It is similar to ordinary water (H₂O), but the hydrogen atoms are replaced by deuterium — a heavier isotope of hydrogen containing one proton and one neutron. Heavy water is not suitable for drinking but is used as a moderator and coolant in nuclear reactors because it slows down neutrons, allowing a controlled nuclear fission reaction. It is also used in scientific research and isotope studies.

Red light has the longest wavelength among visible colours, approximately 620–750 nanometers. Due to its longer wavelength, red light is scattered the least, which is why it is used in traffic signals and as warning lights for better visibility over long distances.

• Blue and Green: Shorter wavelengths, scattered more easily.
• Yellow: Intermediate wavelength between red and green.

Explanation: The sour taste of a lemon is due to the presence of citric acid. Citric acid is a weak organic acid found naturally in citrus fruits such as lemons, oranges, and limes. It gives them their characteristic tangy flavor and also acts as a natural preservative.

Explanation: White Blood Cells (WBCs), also known as leukocytes, act as the body’s defense system. They protect the body against pathogens such as bacteria, viruses, and fungi. Different types of WBCs — like lymphocytes, monocytes, neutrophils, eosinophils, and basophils — have specialized roles in immunity. Because they “fight” infections, they are popularly called the ‘soldiers of the human body.’

A fuse wire has high resistance and a low melting point. It is made of materials like tin-lead alloy so that it melts quickly when excessive current flows through the circuit. This breaks the circuit and prevents damage to electrical appliances due to overheating or short circuits.

• High resistance: Causes heating when excess current passes.
• Low melting point: Ensures the fuse melts easily to interrupt the circuit.

The aorta is the largest artery in the human body. It originates from the left ventricle of the heart and distributes oxygenated blood to all parts of the body through its various branches. The aorta has thick, elastic walls that can withstand the high pressure of blood pumped directly from the heart. It is divided into parts such as the ascending aorta, aortic arch, and descending aorta.

Other options (for context)

• Capillary: The smallest blood vessels where exchange of gases and nutrients occurs.
• Vena cava: The largest vein, not an artery; it carries deoxygenated blood back to the heart.
• Pulmonary vein: Carries oxygenated blood from the lungs to the heart but is not the largest vessel.

Explanation: According to the Bureau of Indian Standards (BIS) – IS 10500:2012, the acceptable limit of total hardness in drinking water, expressed as CaCO₃, is 200 mg/l. The permissible limit in the absence of an alternate source is up to 600 mg/l. Excess hardness can cause scaling and affect the taste of water.

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.

Xylem is a type of vascular tissue in plants responsible for the transportation of water and dissolved minerals from the roots to the stems and leaves. The upward movement of water through xylem vessels occurs due to processes like transpiration pull, root pressure, and capillary action. Xylem also provides mechanical support to the plant.

Other options (for context)

• Food: Transported by phloem, not xylem.
• Nutrient: Minerals are carried along with water but the primary function of xylem is water transport.
• Both food and water: Incorrect — only phloem transports food; xylem transports water.

The Kyoto Protocol is an international agreement related to the environment, specifically aimed at reducing greenhouse gas emissions that contribute to global warming and climate change. It was adopted in Kyoto, Japan, in 1997 and came into force in 2005. The protocol set binding targets for developed countries to limit or reduce their carbon dioxide (CO₂) and other greenhouse gas emissions.

Other options (for context)

• Nuclear Power: Not the focus of the Kyoto Protocol; this relates to energy generation and atomic energy regulation.
• Trade: Covered under agreements like GATT and WTO, not the Kyoto Protocol.
• None of these: Incorrect — the Kyoto Protocol deals specifically with environmental protection.