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.

Explanation: A barometer is an instrument used primarily for measuring atmospheric pressure. It helps in weather forecasting and determining altitudes (since atmospheric pressure decreases with height). A mercury barometer consists of a glass tube filled with mercury inverted in a mercury reservoir. When air pressure changes, the height of the mercury column changes accordingly. However, a barometer cannot measure the density of a liquid — that function is performed by a hydrometer.

Explanation: Dengue fever is caused by the Dengue virus, which belongs to the Flavivirus group. It is transmitted to humans through the bite of an infected Aedes aegypti mosquito. The virus multiplies in the blood and causes symptoms such as high fever, severe headache, joint and muscle pain, rashes, and low platelet count. In severe cases, it can lead to Dengue hemorrhagic fever or Dengue shock syndrome. Prevention includes controlling mosquito breeding and avoiding mosquito bites.

Explanation: The major cause of global warming is the increasing concentration of carbon dioxide (CO₂) in the atmosphere. CO₂ is a greenhouse gas that traps heat radiating from the Earth's surface, preventing it from escaping into space — a phenomenon known as the greenhouse effect. Human activities such as the burning of fossil fuels, deforestation, and industrial emissions have significantly increased CO₂ levels, leading to a rise in the Earth’s average temperature, melting of glaciers, rising sea levels, and climate change.

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: Animals that live in or on trees are called arboreal animals. They are adapted to climbing and moving about among branches. Examples include monkeys, squirrels, chameleons, and tree frogs. Their adaptations may include grasping limbs, prehensile tails, claws, or adhesive pads that help them cling to tree surfaces and move efficiently among branches.

Silver is the best conductor of heat among the given options. It efficiently transfers heat due to the presence of free electrons that move easily within its metallic structure. Because of this property, silver is used in high-quality thermal equipment and electrical contacts.

• Mercury: A metal but conducts heat less effectively than silver.
• Leather and Benzene: Poor conductors of heat (insulators).