Explanation: The human retina contains two types of photoreceptor cells — rods and cones. Cones are concentrated in the central part of the retina (fovea) and are sensitive to different wavelengths of light — red, green, and blue. These enable colour vision and visual acuity in bright light. Rods, on the other hand, function in dim light and provide black-and-white vision.

Biogas is a source of non-conventional (renewable) energy. It is produced through the anaerobic decomposition of organic waste materials such as animal dung, crop residues, and kitchen waste. Biogas mainly contains methane (CH₄) and is used as a clean fuel for cooking, lighting, and even power generation. It helps reduce dependence on fossil fuels and minimizes environmental pollution.

Other options (for context)

• Firewood: A traditional biomass fuel, not classified as a modern non-conventional source.
• Diesel: A conventional fossil fuel derived from crude oil.
• Petrol: Another conventional fossil fuel obtained from petroleum refining.

Explanation: The femur is the single bone of the thigh region and extends from the hip joint to the knee joint. It supports the body’s weight during standing, walking, and running. The femur is the longest, heaviest, and strongest bone in the human skeleton. It articulates with the pelvic girdle at its proximal end (head of femur fits into the acetabulum of the pelvis) and with the tibia and patella at the distal end.

Average Length: Approximately 45 cm (about 1/4 of body height).

Dynamite was invented by Alfred Nobel in 1867. He was a Swedish chemist, engineer, and industrialist who discovered that nitroglycerin could be stabilized by mixing it with an absorbent material like diatomaceous earth, making it safer to handle and transport. This invention revolutionized the mining and construction industries. In his later years, Alfred Nobel established the Nobel Prizes using the wealth he accumulated from his inventions.

Other options (for context)

• J. B. Dunlop: Invented the pneumatic rubber tyre.
• James Simons: A mathematician and hedge fund manager, not an inventor.
• Peter Hargreaves: Not related to any major scientific invention; possibly confused with James Hargreaves, who invented the Spinning Jenny.

Explanation: To read small letters, a convex lens (converging lens) with a short focal length such as 5 cm is used as a magnifying glass. It produces an enlarged, upright, and virtual image of the object, making tiny text appear bigger and clearer to the eye.

Explanation: Aerosols (such as those used in spray cans, refrigerants, and air conditioners) often contain chlorofluorocarbons (CFCs) and other halogen compounds. These chemicals rise to the stratosphere, where ultraviolet (UV) radiation breaks them down, releasing chlorine atoms. Each chlorine atom can destroy thousands of ozone (O₃) molecules, leading to ozone layer depletion. The thinning of the ozone layer allows more harmful UV rays to reach Earth, increasing the risk of skin cancer, eye damage, and affecting ecosystems.

Explanation: Hardware refers to the physical components of a computer that can be seen and touched, such as the monitor, keyboard, mouse, CPU, and printer. In contrast, software consists of programs and instructions that control the hardware and perform tasks.

Red Blood Corpuscles (RBCs), also known as erythrocytes, are formed in the bone marrow. Specifically, they are produced in the red bone marrow of large bones through a process called erythropoiesis. The hormone erythropoietin, secreted mainly by the kidneys, regulates this process. Mature RBCs are responsible for transporting oxygen from the lungs to body tissues using the pigment hemoglobin.

Other options (for context)

• Heart: Pumps blood but does not produce blood cells.
• Liver: In adults, it does not form RBCs (though it does in the fetal stage).
• Kidney: Produces erythropoietin hormone but not RBCs directly.

Explanation: Although most non-metals do not conduct electricity, graphite is an exception. In graphite, each carbon atom is bonded to three others, leaving one free electron that moves freely between layers, allowing it to conduct electricity efficiently.

Explanation: Smallpox is a highly contagious and deadly disease caused by the Variola virus. It is characterized by high fever, fatigue, and a distinctive pustular rash on the skin. Smallpox was one of the most devastating diseases in human history, but thanks to the worldwide vaccination campaign led by the World Health Organization (WHO), it was officially eradicated in 1980. The vaccine for smallpox was first developed by Edward Jenner in 1796.