Explanation: The main cause of global warming is the rise in the concentration of carbon dioxide (CO₂) and other greenhouse gases in the atmosphere. These gases trap heat radiated from the Earth’s surface, creating the greenhouse effect and leading to an overall increase in the planet’s temperature.

Explanation: The gas used for artificial ripening of green fruits is ethylene (C₂H₄). Ethylene is a natural plant hormone that regulates fruit ripening by stimulating the conversion of starches into sugars, making the fruit softer and sweeter. In artificial ripening, ethylene gas is introduced in controlled amounts to uniformly ripen fruits such as bananas, mangoes, and tomatoes. However, the use of harmful chemicals like calcium carbide for ripening is banned because it produces toxic substances like acetylene.

Explanation: Vitamin A is essential for maintaining healthy vision, skin, and immune function. A deficiency of Vitamin A leads to night blindness (nyctalopia), a condition in which a person has difficulty seeing in dim light or at night. In severe cases, it can cause xerophthalmia (dryness of the cornea) and even blindness. Vitamin A is obtained from foods such as carrots, milk, butter, fish liver oil, and green leafy vegetables.

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: The elements Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) — collectively known as CHON — were primarily responsible for the origin of life on Earth. These elements are the basic building blocks of all biological molecules such as proteins, nucleic acids, carbohydrates, and lipids. Early Earth's atmosphere contained these elements in gaseous forms (like CH₄, NH₃, H₂, and H₂O), which combined under lightning and UV radiation to form simple organic compounds — a process explained by the Miller–Urey experiment.

Explanation: Amniocentesis is a medical diagnostic procedure used to determine the genetic and chromosomal makeup of a developing fetus. It involves the withdrawal of a small amount of amniotic fluid from the uterus using a fine needle. This fluid contains fetal cells that can be tested to identify genetic disorders such as Down’s syndrome and also to determine the sex of the fetus. However, using amniocentesis for sex determination is illegal in India under the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act to prevent female foeticide.

Explanation: Asbestos is a group of naturally occurring fibrous silicate minerals once widely used in construction and insulation. When inhaled, asbestos fibers accumulate in the lungs and pleura, leading to inflammation and scarring over years. This chronic exposure can cause asbestosis (lung fibrosis) and mesothelioma — a deadly cancer of the membrane lining the lungs (pleura) or abdomen (peritoneum).

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.

Neil Armstrong was the first man to land on the Moon on July 20, 1969, during NASA’s Apollo 11 mission. He became the first human to step onto the lunar surface, famously saying, "That's one small step for man, one giant leap for mankind." His historic achievement marked a major milestone in space exploration.

Other options (for context)

• Aldrin: Refers to Buzz Aldrin, the second man to walk on the Moon, who accompanied Armstrong on Apollo 11.
• Michael Collins: The third member of the Apollo 11 crew, who orbited the Moon in the command module but did not land.
• Gagarin: Refers to Yuri Gagarin, the first human to travel into space in 1961, but he never landed on the Moon.

Argon (Ar) is an inert gas, also known as a noble gas. It belongs to Group 18 of the periodic table, whose elements are characterized by complete outer electron shells, making them chemically unreactive. Argon constitutes about 0.93% of the Earth's atmosphere and is widely used in electric bulbs, welding, and in creating inert environments for chemical reactions.

Other options (for context)

• Hydrogen: Highly reactive and combustible gas.
• Nitrogen: Relatively less reactive but not inert; forms compounds like ammonia.
• Oxygen: Highly reactive gas essential for combustion and respiration.