Explanation: The colour of the sky without an atmosphere would appear black. On Earth, the sky looks blue because molecules in the atmosphere scatter sunlight — a phenomenon called Rayleigh scattering. Shorter wavelengths of light (blue and violet) are scattered more than longer wavelengths (red). However, if there were no atmosphere to scatter sunlight, no light would be diffused in all directions, and the sky would appear completely dark or black, just as it does from the surface of the Moon or in outer space.

Aluminium (Al) is the most abundant metal in the Earth's crust, making up about 8.2% by weight. It occurs mainly in the form of bauxite ore (Al₂O₃·2H₂O). Despite being highly abundant, aluminium is never found in its pure form naturally due to its strong reactivity with oxygen. It is widely used in industries for making utensils, aircraft parts, electrical wires, and packaging materials.

Other options (for context)

• Zinc: Found in smaller quantities; used in galvanization and alloys.
• Copper: Less abundant; mainly used in electrical wiring.
• Iron: Very abundant but ranks below aluminium in crustal abundance.

The CPU (Central Processing Unit) is known as the brain of the computer. It performs all major operations such as processing data, executing instructions, and controlling other components. The CPU consists of two main parts — the Arithmetic Logic Unit (ALU) for calculations and the Control Unit (CU) for directing operations.

• ALU: Handles arithmetic and logic operations only.
• Memory: Stores data and instructions, but doesn’t process them.
• Control Unit: Directs the flow of data but works as part of the CPU.

Explanation: Sound waves are not electromagnetic waves; they are mechanical waves that require a material medium (like air, water, or solids) for propagation. In contrast, radio waves, X-rays, and visible light are electromagnetic waves that can travel through a vacuum.

Explanation: Vitamin K is produced by gut bacteria (Escherichia coli) in the large intestine. It plays a vital role in the synthesis of prothrombin, a protein essential for normal blood clotting. Although some of it is obtained from food (like leafy vegetables), intestinal bacteria supply a significant portion of the body’s requirement.

Explanation: The disease AIDS (Acquired Immuno Deficiency Syndrome) is caused by the HIV (Human Immunodeficiency Virus). HIV attacks the body’s immune system, specifically the CD4 or T-helper cells, weakening the body’s ability to fight infections and diseases. It spreads through unprotected sexual contact, sharing of infected needles, transfusion of contaminated blood, or from an infected mother to her child during birth or breastfeeding. There is no permanent cure, but antiretroviral therapy (ART) helps control the virus and prolong life.

Explanation: Albert Einstein received the Nobel Prize in Physics in 1921 for his discovery of the photoelectric effect, not for his famous theory of relativity. In the photoelectric effect, when light of a certain frequency falls on a metal surface, it emits electrons. Einstein explained this phenomenon by proposing that light is made up of discrete packets of energy called photons. The energy of each photon is given by E = hν, where h is Planck’s constant and ν is the frequency of light. This discovery laid the foundation for quantum physics and modern electronics.

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.