Explanation: Nitrogen is an essential nutrient for plants, used in the synthesis of amino acids, proteins, nucleic acids, and chlorophyll. Plants absorb nitrogen from the soil primarily in the inorganic forms of nitrate (NO₃⁻) and nitrite (NO₂⁻). Urea, when applied as fertilizer, is first converted by soil bacteria into ammonia (NH₃) and then into nitrates through nitrification before being absorbed by roots.

Key Steps:

• Ammonification: Conversion of organic nitrogen (from dead plants/animals) into ammonia.
• Nitrification: Conversion of ammonia into nitrites and then nitrates by bacteria like Nitrosomonas and Nitrobacter.
• Assimilation: Uptake of nitrates by plants to form amino acids and proteins.

William Harvey, an English physician, discovered the process of blood circulation in 1628. He demonstrated that the heart acts as a pump to circulate blood continuously throughout the body via a closed system of arteries and veins. His work, published in "De Motu Cordis" (On the Motion of the Heart and Blood), laid the foundation for modern cardiovascular physiology.

Other options (for context)

• Mary Anderson: Invented the windshield wiper, not related to medicine.
• Virginia Apgar: Developed the Apgar score for assessing newborn health.
• Robert Feulgen: Discovered a staining technique for DNA, not blood circulation.

Explanation: The angle of friction and the angle of repose are equal to each other. The angle of friction is the angle made by the resultant of the limiting friction and normal reaction with the normal force, while the angle of repose is the maximum angle of an inclined plane at which an object just begins to slide. Both represent the same physical condition of impending motion.

Explanation: The acid present in the human stomach is hydrochloric acid (HCl). It helps in the digestion of food by breaking down proteins and activating the enzyme pepsin. HCl also kills harmful microorganisms present in the food, aiding in maintaining a healthy digestive system.

Explanation: The sex of a newborn baby is determined by the chromosome inherited from the father. Humans have 23 pairs of chromosomes — 22 pairs of autosomes and one pair of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The mother always contributes an X chromosome, but the father can contribute either an X or a Y. If the sperm carrying an X chromosome fertilizes the ovum, the child will be a girl (XX); if the sperm carries a Y chromosome, the child will be a boy (XY).

Explanation: Acid rain is caused by the presence of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) in the atmosphere. These gases are mainly released from the burning of coal, petroleum, and other fossil fuels in industries and vehicles. When they combine with water vapor and oxygen in the air, they form sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These acids mix with rainwater and fall as acid rain, which damages soil, crops, aquatic life, and even monuments like the Taj Mahal.

Explanation: Measles is a highly contagious disease caused by the Measles virus, which belongs to the Paramyxoviridae family, genus Morbillivirus. It spreads through respiratory droplets from coughing or sneezing by an infected person. Common symptoms include high fever, cough, runny nose, red eyes, and a characteristic red skin rash. Severe cases can lead to pneumonia or encephalitis. Measles can be effectively prevented by the MMR vaccine (Measles, Mumps, and Rubella).

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.