The Abacus is considered the first calculating device in human history. It was invented around 2500 BCE and used in ancient civilizations such as China, Egypt, and Mesopotamia. The Abacus consists of beads that slide on rods or wires, representing numbers, and is used to perform basic arithmetic operations like addition, subtraction, multiplication, and division. It laid the foundation for modern computing and calculating tools.

Other options (for context)

• Calculator: A modern electronic device for computation, developed much later.
• Turing machine: A theoretical computational model proposed by Alan Turing in 1936.
• Pascaline: A mechanical calculator invented by Blaise Pascal in 1642, long after the Abacus.

Explanation: Nucleons are the particles that make up the nucleus of an atom — namely, protons and neutrons. A positron is not a nucleon; it is the antiparticle of the electron, having the same mass as an electron but a positive charge, and exists outside the nucleus.

Explanation: Fire extinguishers commonly used in houses and offices contain water, potassium bicarbonate (KHCO₃), and fluorocarbons. These chemicals help in extinguishing fires by cooling, cutting off oxygen supply, and preventing the spread of flames without leaving harmful residues.

Explanation: Proteins are complex organic compounds composed of smaller units called amino acids. Each amino acid contains an amino group (–NH₂), which contributes nitrogen to the molecular structure. This makes nitrogen an essential element in all proteins. Carbohydrates and fats lack nitrogen, while only some vitamins contain it.

Explanation: When ice floats in water, it displaces an amount of water equal to the weight of the ice. As the ice melts, it converts into the same amount of water, exactly filling the displaced volume. Therefore, the water level remains unchanged after all the ice melts.

Insecticides primarily act upon the nervous system of insects. They interfere with the transmission of nerve impulses, leading to paralysis and ultimately death. Common insecticides like organophosphates and carbamates inhibit the enzyme acetylcholinesterase, causing continuous nerve stimulation. Some others, like pyrethroids, disrupt sodium channel functioning in nerve cells.

Other options (for context)

• Muscular system: Indirectly affected due to nerve failure, but not the primary target.
• Digestive system: Rarely affected — insecticides mainly disrupt neural signaling, not digestion.

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).

Explanation: Rakesh Sharma was the first Indian astronaut to travel into space. He flew aboard the Soviet spacecraft Soyuz T-11 on April 3, 1984, as part of a joint mission between ISRO (Indian Space Research Organisation) and the Soviet Intercosmos program. During his space mission, he spent about 8 days aboard the Salyut-7 space station. When asked by then Prime Minister Indira Gandhi how India looked from space, he famously replied, “Saare Jahan Se Achha.”