Explanation: A modem (short for modulator-demodulator) is a device that enables computers to communicate with each other over telephone lines. It converts digital signals from a computer into analog signals for transmission and then back into digital form at the receiving end.

The ozone hole leads to an increase in ultraviolet (UV) radiation reaching the Earth's surface. The depletion of the ozone layer, caused by chemicals like CFCs (chlorofluorocarbons), reduces its ability to block harmful UV rays. This can result in health issues such as skin cancer, cataracts, and also harm to crops and marine ecosystems.

• Acid rain: Caused by sulphur and nitrogen oxides, not ozone depletion.
• Global warming: Caused by greenhouse gases trapping heat.
• Greenhouse effect: Natural process intensified by CO₂, not related to ozone loss.

Graphite allows electric current to pass through it. Although it is a form of carbon, its atoms are arranged in layers with free-moving electrons, making it a good conductor of electricity. This property makes graphite useful in electrodes, batteries, and brushes of electric motors.

• Glass, Rubber, PVC: All are insulators and do not conduct electricity.

Explanation: Ringworm (also known as Dermatophytosis or Tinea infection) is a fungal skin disease. It appears as red, circular, itchy patches on the skin, scalp, or nails. Despite its name, it is not caused by a worm but by a fungus that grows on keratinized tissues such as skin, hair, and nails. The disease spreads through direct contact with infected individuals, animals, or contaminated objects.

Common Types:

• Tinea capitis: Ringworm of the scalp
• Tinea corporis: Ringworm of the body
• Tinea pedis: Athlete’s foot (infection of feet)
• Tinea cruris: Jock itch (infection of groin area)

The Abacus is considered the first calculating device in human history. It was invented around 2500 BCE and used in ancient civilizations such as China, Egypt, and Mesopotamia. The Abacus consists of beads that slide on rods or wires, representing numbers, and is used to perform basic arithmetic operations like addition, subtraction, multiplication, and division. It laid the foundation for modern computing and calculating tools.

Other options (for context)

• Calculator: A modern electronic device for computation, developed much later.
• Turing machine: A theoretical computational model proposed by Alan Turing in 1936.
• Pascaline: A mechanical calculator invented by Blaise Pascal in 1642, long after the Abacus.

Explanation: Carbon monoxide (CO) is a colorless, odorless, and highly poisonous gas. When inhaled, it competes with oxygen for binding sites on the haemoglobin molecule in red blood cells. CO has approximately 200–250 times greater affinity for haemoglobin than oxygen. As a result, it forms a stable compound called carboxyhaemoglobin (HbCO).

This compound prevents haemoglobin from carrying oxygen to body tissues, leading to oxygen deprivation (hypoxia). Prolonged exposure to CO can cause headache, dizziness, nausea, unconsciousness, and even death due to suffocation.

Chemical Reaction:

Hb + CO → HbCO (Carboxyhaemoglobin)

Explanation: The pyloric sphincter is a circular band of smooth muscle located at the junction of the stomach and the duodenum (first part of the small intestine). It regulates the passage of partially digested food (chyme) from the stomach into the small intestine and prevents backflow. The cardiac sphincter, by contrast, controls food entry into the stomach from the esophagus.

Explanation: Ampicillin is a semi-synthetic derivative of penicillin and belongs to the β-lactam group of antibiotics. It inhibits bacterial cell wall synthesis and is effective against a wide range of gram-positive and gram-negative bacteria. Therefore, it is classified as a broad-spectrum antibiotic. Paracetamol is not an antibiotic, and penicillin is a narrow-spectrum antibiotic.