Explanation: The hard structure of bones and teeth is primarily made of calcium phosphate in the crystalline form of tricalcium phosphate (Ca₃(PO₄)₂). It provides rigidity and strength. In tooth enamel, a small portion is converted into fluorapatite, which makes teeth resistant to bacterial decay.

Explanation: The sour taste of a lemon is due to the presence of citric acid. Citric acid is a weak organic acid found naturally in citrus fruits such as lemons, oranges, and limes. It gives them their characteristic tangy flavor and also acts as a natural preservative.

The Itai-Itai disease is caused by Cadmium (Cd) poisoning. The disease was first identified in Japan, where industrial discharge of cadmium-contaminated water led to severe pollution. People consuming rice grown in this contaminated water suffered from extreme pain in bones and joints, along with kidney damage. The name "Itai-Itai" literally means "ouch-ouch" in Japanese, reflecting the severe pain experienced by victims.

Other options (for context)

• Nickel: Causes dermatitis and allergic reactions but not Itai-Itai disease.
• Chromium: Can cause skin and respiratory problems, not bone-related disorders.
• Mercury: Leads to Minamata disease, not Itai-Itai disease.

Explanation: Smallpox is a highly contagious and deadly disease caused by the Variola virus. It is characterized by high fever, fatigue, and a distinctive pustular rash on the skin. Smallpox was one of the most devastating diseases in human history, but thanks to the worldwide vaccination campaign led by the World Health Organization (WHO), it was officially eradicated in 1980. The vaccine for smallpox was first developed by Edward Jenner in 1796.

Explanation: In fused (molten) sodium chloride, electric current passes due to the presence of free ions. When solid NaCl is melted, it dissociates into Na⁺ and Cl⁻ ions, which can move freely and conduct electricity. In the solid state, these ions are fixed in the crystal lattice, so it does not conduct electricity.

Explanation: Vitamin B₁₂, also known as Cobalamin, is the only vitamin that contains a metal ion — cobalt (Co) — at its core. It plays a crucial role in red blood cell formation, DNA synthesis, and proper nervous system function. Deficiency of Vitamin B₁₂ leads to pernicious anemia and neurological disorders. It is found mainly in animal-based foods such as meat, eggs, milk, and fish, and is synthesized by certain bacteria.

The Kyoto Protocol is an international agreement related to the environment, specifically aimed at reducing greenhouse gas emissions that contribute to global warming and climate change. It was adopted in Kyoto, Japan, in 1997 and came into force in 2005. The protocol set binding targets for developed countries to limit or reduce their carbon dioxide (CO₂) and other greenhouse gas emissions.

Other options (for context)

• Nuclear Power: Not the focus of the Kyoto Protocol; this relates to energy generation and atomic energy regulation.
• Trade: Covered under agreements like GATT and WTO, not the Kyoto Protocol.
• None of these: Incorrect — the Kyoto Protocol deals specifically with environmental protection.

The Red Data Book contains information about endangered and threatened species of plants and animals. It is published by the International Union for Conservation of Nature (IUCN). The book categorizes species based on their risk of extinction, such as Critically Endangered, Endangered, Vulnerable, and Extinct. It serves as a global reference for monitoring biodiversity conservation and guiding environmental protection policies.

Other options (for context)

• All plant species: Incorrect — it includes only those plants that are endangered or threatened.
• All animal species: Incorrect — it covers only the endangered and vulnerable ones, not all animals.
• All extinct species: Incorrect — it includes species at risk of extinction, not only those already extinct.

Explanation: Tuberculosis (TB) is not a viral disease; it is caused by a bacterium called Mycobacterium tuberculosis. In contrast, Polio, Swine flu, and Hepatitis are viral diseases caused by different types of viruses.