Everyday Science

Gun metal is a type of bronze alloy traditionally composed of copper, tin, and zinc. This combination provides good corrosion resistance and strength, making option D correct.

Argon is the third most common gas in Earth's atmosphere after nitrogen and oxygen, making it the correct answer. Carbon dioxide is present in much smaller amounts, and hydrogen and neon are trace gases.

The stethoscope was invented by René Laennec in 1816, making option A the correct answer. The other options are inventors of different technologies unrelated to the stethoscope.

The human heart has four chambers: two atria and two ventricles. This structure allows for efficient circulation of oxygenated and deoxygenated blood.

The Solar System as a concept was not 'discovered' by a single person but understood progressively over centuries. The geocentric model (Earth-centered) was dominant for many centuries until Nicolaus Copernicus formulated the heliocentric model in 1543, proposing that the Sun, not the Earth, is at the center of the planetary system. This was a foundational shift in understanding the structure of the Solar System. Later, Galileo Galilei's telescopic observations provided strong empirical evidence supporting the heliocentric model. Therefore, while Copernicus did not 'discover' the Solar System, he is credited with the first comprehensive heliocentric model that correctly described its structure, making option A (Copernicus) the best answer among the choices given.

A chronometer is a precise instrument used to measure time, especially in navigation and scientific contexts. Therefore, the correct answer is 'Time'.

Sodium stearate is a common component of soap, formed by the saponification of stearic acid with sodium hydroxide. The other options are salts but are not components of soap.

Deficiency diseases occur when the body lacks essential nutrients, particularly vitamins, which are crucial for various bodily functions. Lack of vitamins leads to specific deficiency diseases such as scurvy (vitamin C deficiency) and rickets (vitamin D deficiency).

Day and night occur because the Earth rotates around its own axis, causing different parts of the Earth to face the Sun or move away from it, resulting in daylight and darkness respectively.

Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, and it is commonly referred to as cast iron due to its high carbon content and brittleness. Therefore, 'Cast iron' is the correct answer.