Explanation: The main constituents of biogas are methane (CH₄) and carbon dioxide (CO₂). Biogas is produced through the anaerobic decomposition of organic matter such as animal dung and plant waste. Methane acts as the primary combustible component, making biogas an efficient and renewable source of energy.

Explanation: The diseases Cholera, Tuberculosis, and Typhoid are all caused by bacteria, whereas Smallpox is caused by a virus (Variola virus). Bacterial diseases are typically treated with antibiotics, while viral diseases require vaccines or antiviral drugs. Cholera spreads through contaminated water, Typhoid through food and water, and Tuberculosis through droplets from infected individuals.

Explanation: Seismology is the branch of geophysics that deals with the study of earthquakes and the propagation of seismic waves through the Earth. Scientists who study earthquakes are called seismologists. Seismology helps in understanding the origin, intensity, and effects of earthquakes, as well as the internal structure of the Earth. The intensity of an earthquake is measured using instruments like the seismograph and scales such as the Richter scale or Moment Magnitude scale.

The full form of LAN is Local Area Network. It is a network that connects computers and devices within a limited geographical area such as a home, office, school, or building. LANs enable data sharing, resource sharing (like printers or internet connections), and communication between connected devices. Common technologies used in LANs include Ethernet and Wi-Fi.

Other options (for context)

• Line Area Network: Incorrect — not a recognized networking term.
• Linear Area Network: Incorrect — there is no such standard network type.
• Land Area Network: Incorrect — misinterpretation of LAN abbreviation.

Aryabhatta was a renowned mathematician and astronomer of ancient India. He lived during the Gupta period (around 476–550 CE) and authored the famous work Aryabhatiya. Aryabhatta introduced the concept of zero (0) and explained the place value system. He also proposed that the Earth rotates on its axis and correctly explained solar and lunar eclipses.

Other options (for context)

• Doctor: No historical evidence of Aryabhatta being a physician.
• Poet: Although his works were written in verse form, his main field was mathematics and astronomy.
• Physicist: The term did not exist in his time; however, his scientific thinking laid foundations for later studies.

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: The nerve cell or neuron is the longest cell in animals. It consists of a cell body, dendrites, and a long fiber called the axon. In humans, the axon of a motor neuron can be over 1 meter long (from the spinal cord to the toes). This structure helps transmit electrical impulses rapidly across the body, allowing coordination and communication between organs.

Explanation: Bilirubin and Biliverdin are yellow and green bile pigments respectively, found in the bile secreted by the liver. These pigments are produced during the breakdown of hemoglobin from old red blood cells (RBCs) in the liver and spleen. Biliverdin (green pigment) is first formed from the heme part of hemoglobin and is then reduced to bilirubin (yellow pigment). The bile containing these pigments helps in the digestion and emulsification of fats. Excess bilirubin in the blood causes jaundice, leading to a yellowing of the skin and eyes.