Explanation: The Sinoatrial (SA) node is a specialized group of modified cardiac muscle fibers located in the right atrium of the heart, near the opening of the superior vena cava. It generates electrical impulses (action potentials) that initiate each heartbeat. Therefore, it is known as the natural pacemaker of the heart.

The impulses from the SA node spread across the atria, causing them to contract, and are then transmitted to the Atrioventricular (AV) node, Bundle of His, and Purkinje fibers to coordinate ventricular contraction.

Normal rate of SA node firing: ~70–75 impulses per minute (in a healthy adult at rest).

In case of SA node failure: Artificial pacemakers are implanted to maintain normal rhythm.

Explanation: Bronchitis is an inflammation of the bronchial tubes, which are the air passages of the respiratory tract that carry air to and from the lungs. The condition causes coughing, mucus production, chest discomfort, and difficulty in breathing. It can be caused by viral or bacterial infections, long-term exposure to smoke, dust, or air pollutants. Chronic bronchitis is a serious form often associated with smoking and is part of a group of lung diseases known as COPD (Chronic Obstructive Pulmonary Disease).

India’s first successful atomic device was tested at Pokhran in the Thar Desert of Rajasthan on 18th May 1974. The operation was code-named "Smiling Buddha" and was conducted by the Bhabha Atomic Research Centre (BARC) under the leadership of scientists like Dr. Raja Ramanna. This event marked India’s entry into the group of nuclear-capable nations.

Other options (for context)

• Bikaner: A city in Rajasthan, but no nuclear tests were conducted there.
• Barmer: Located near the Pakistan border, not the site of India’s nuclear test.
• Jaisalmer: Close to Pokhran geographically, but not the test location.

Explanation: The elements Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) — collectively known as CHON — were primarily responsible for the origin of life on Earth. These elements are the basic building blocks of all biological molecules such as proteins, nucleic acids, carbohydrates, and lipids. Early Earth's atmosphere contained these elements in gaseous forms (like CH₄, NH₃, H₂, and H₂O), which combined under lightning and UV radiation to form simple organic compounds — a process explained by the Miller–Urey experiment.

Metrology is the science of measurement. It deals with the establishment of units of measurement, their standards, and the development of new measuring methods and instruments. Metrology ensures accuracy and consistency in measurements across scientific research, engineering, manufacturing, and trade. It is broadly classified into three types — scientific metrology, industrial metrology, and legal metrology.

Other options (for context)

• Weather: The study of weather and atmosphere is called Meteorology, not Metrology.
• Distance: Measuring distance is just one aspect of metrology, not its entire scope.
• None of these: Incorrect — metrology specifically refers to measurement science.

Deciduous trees are those that shed all their leaves once a year, usually during the dry or winter season to conserve water and energy. After a period of dormancy, they grow new leaves at the beginning of the next growing season. Common examples include teak, sal, neem, and peepal. This adaptation helps the trees survive unfavorable climatic conditions.

Other options (for context)

• Coniferous trees: Mostly evergreen and do not shed all their leaves at once; examples include pine and deodar.
• Evergreen trees: Retain their leaves throughout the year and shed them gradually.
• Both deciduous and coniferous trees: Incorrect — only deciduous trees completely shed their leaves annually.

Explanation: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

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: Sound waves with frequencies below 20 Hz are called infrasonic waves. These waves are inaudible to the human ear but can be produced by natural phenomena such as earthquakes, volcanic eruptions, and ocean waves.