The normal hearing range of human beings is from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasonic sounds, while those above 20,000 Hz are known as ultrasonic sounds, both of which are inaudible to humans. With age or prolonged exposure to loud noise, the upper hearing limit tends to decrease, reducing sensitivity to high-frequency sounds.

Other options (for context)

• 30 to 30,000 Hz: Incorrect — exceeds the upper audible limit for humans.
• 50 to 50,000 Hz: Incorrect — animals like dogs and bats can hear this range, not humans.
• 40 to 40,000 Hz: Incorrect — the human ear cannot detect frequencies that high.

Explanation: Humans first landed on the Moon on July 20, 1969, during NASA’s Apollo 11 mission. American astronauts Neil Armstrong and Edwin “Buzz” Aldrin became the first and second humans to walk on the Moon, while Michael Collins orbited above in the command module. Neil Armstrong’s famous words, “That’s one small step for man, one giant leap for mankind,” marked this historic achievement in space exploration.

Explanation: Nitrogen (N₂) constitutes the major portion of the Earth's atmosphere by volume — about 78%. It is a colorless, odorless, and inert gas that plays a vital role in maintaining atmospheric stability and supporting life through the nitrogen cycle. Oxygen makes up about 21% of the atmosphere and is essential for respiration and combustion. Other gases like argon (~0.93%) and carbon dioxide (~0.04%) are present in much smaller amounts.

Explanation: An artery is a large blood vessel that carries blood away from the heart to various parts of the body. Arteries have thick, elastic walls to withstand high pressure. In contrast, veins carry blood toward the heart, and capillaries are tiny vessels that connect arteries and veins.

Explanation: Aurora Borealis, also known as the Northern Lights, is a natural light phenomenon visible mainly in the northern hemisphere, near the Arctic regions. It occurs when charged particles from the solar wind interact with gases like oxygen and nitrogen in the Earth's upper atmosphere (ionosphere). These collisions release energy in the form of colorful lights, usually green, pink, red, or violet. A similar phenomenon in the southern hemisphere is called the Aurora Australis or Southern Lights.

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: The longest day in the Southern Hemisphere occurs on 22nd December, which marks the Summer Solstice in that hemisphere. On this date, the South Pole is tilted toward the Sun, resulting in longer daylight hours and shorter nights. Conversely, at the same time, the Northern Hemisphere experiences its winter solstice—the shortest day and longest night of the year. After 22nd December, the days in the Southern Hemisphere gradually begin to shorten.

Explanation: The age of a tree is determined by counting the annual growth rings (also called annual rings) visible in a cross-section of its trunk. Each ring consists of a light band (spring wood) and a dark band (autumn wood), representing one year of growth. The study of these rings to determine a tree’s age and past climate conditions is known as dendrochronology.