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.

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.

The Widal test is used to diagnose Typhoid fever, a bacterial infection caused by Salmonella typhi. The test detects the presence of agglutinating antibodies (O and H) in a patient’s serum against the antigens of the typhoid bacteria. It helps confirm infection during the second week of illness.

• Cholera: Diagnosed by stool culture for Vibrio cholerae.
• Tuberculosis: Detected using sputum test or Mantoux test.
• AIDS: Diagnosed through ELISA or Western blot test for HIV.

Explanation: A transformer is an electrical device used to increase (step-up) or decrease (step-down) the voltage of alternating current (A.C.). It works on the principle of electromagnetic induction and functions only with A.C., not with D.C. A transformer consists of two coils—primary and secondary—wound around a soft iron core. The voltage ratio between these coils depends on the number of turns in each winding. Transformers are used in power transmission, distribution, and electronic devices to regulate voltage levels efficiently.

Explanation: The audible range of sound refers to the frequencies that can be detected by the average human ear, typically from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasonic and those above 20,000 Hz are called ultrasonic. Hearing sensitivity decreases with age, and most adults can hear up to about 16,000 Hz.

Glaciers usually melt first at the bottom. The pressure at the base of a glacier is very high, which lowers the melting point of ice — a phenomenon known as pressure melting. Additionally, heat from the Earth’s crust and friction caused by glacier movement also contribute to faster melting at the base.

• Top surface: Exposed to air but melts slower due to lower pressure.
• Sides and middle surface: Experience less pressure and friction, so melting is slower.

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.