Everyday Science

Nitrogen is the principal element of the Earth's atmosphere, making up about 78% of it by volume, which is why option B is correct. Oxygen is the second most abundant, but not the principal element.

Elements in the periodic table are arranged based on their atomic number, which is the number of protons in the nucleus of an atom. This arrangement reflects the periodicity of element properties.

There are two main models for primary colors: the traditional pigment model (RYB) and the RGB model used for light. The traditional primary colors in art and pigment mixing are red, yellow, and blue (RYB). However, in the RGB color model used for digital screens and light, the primary colors are red, green, and blue. Since the question does not specify the model, option A (red, blue, green) correctly identifies the primary colors in the RGB model, which is commonly taught in science and digital contexts. The other options list incorrect combinations or colors not considered primary in either model.

The average adult human body contains approximately 5 liters of blood, which supports circulation and oxygen transport throughout the body.

Hydrogen is the most abundant element in the universe, making up about 75% of its elemental mass. It is the primary building block of stars and galaxies, far more abundant than nitrogen or oxygen.

A pyrometer is an instrument used to measure very high temperatures, typically in industrial settings, where conventional thermometers cannot be used. Hygrometers measure humidity, altimeters measure altitude, and 'None of these' is incorrect.

Salt is commonly mixed with ice on roads in cold countries because it lowers the freezing point of water, causing ice to melt and preventing slipping. Sugar and nitrogen do not have this effect, and "None of these" is incorrect.

AIDS (caused by HIV) is transmitted through specific body fluids such as blood, semen, vaginal fluids, rectal fluids, and breast milk. It spreads primarily through unprotected sexual contact, sharing needles, and from mother to child during pregnancy, childbirth, or breastfeeding. It is not spread through air, water, or casual touch such as hugging or shaking hands. Therefore, none of the options A (Air), B (Water), or C (Touch) are correct modes of transmission. The correct answer is D (None of these).

Mercury is the smallest planet in the Solar System by diameter among the given options. Jupiter is the largest, Earth is larger than Mercury, and 'None of these' is incorrect since Mercury is indeed the smallest.

In 2016, estimates suggested that the kangaroo population in Australia was about 44 million, which is roughly double the human population of 24 million at that time. This makes option C the correct answer.