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.

Explanation: The pale yellow color of normal human urine is due to the presence of a pigment called Urochrome. It is produced from the breakdown of hemoglobin in red blood cells. The intensity of the color depends on the concentration of the urine — darker yellow indicates concentrated urine, while very pale or colorless urine suggests dilution due to high water intake.

Explanation: Execution time is the period during which a job or program is actively processed by the computer’s CPU. It refers to the actual time taken to execute instructions, excluding waiting or idle times such as input/output delays or queue time.

Explanation: The Curie point (or Curie temperature) is the temperature above which a ferromagnetic material loses its magnetic properties and becomes paramagnetic. At this point, thermal energy overcomes the magnetic alignment of atoms. For example, the Curie temperature of iron is about 770°C.

Explanation: Humidity is a measure of the amount of water vapour (moisture) present in the air. It indicates how saturated the air is with moisture and affects weather, comfort, and precipitation. High humidity means the air contains more water vapour, while low humidity indicates dry air.

The O blood group, specifically O negative (O−), is considered the Universal Donor because it can be safely transfused to people of all other blood groups. This is due to the absence of A and B antigens on red blood cells, minimizing the risk of immune reactions during transfusion.

• A: Can donate only to A and AB groups.
• AB: Universal recipient, not donor.
• B: Can donate only to B and AB groups.

All of the above are examples of static electricity. Static electricity is the result of the accumulation of electric charges on the surface of objects. When materials like glass, silk, ebonite, or wool are rubbed together, electrons are transferred, creating a charge imbalance. Similarly, lightning is a large-scale natural discharge of static electricity between clouds and the Earth.

• Glass rod & silk cloth: Positive and negative charges develop.
• Ebonite rod & wool: Negative charge accumulates on ebonite.
• Lightning: Discharge of atmospheric static electricity.

Explanation: The main function of the kidneys is to purify blood by removing waste products, excess salts, and extra water from the body in the form of urine. Each kidney contains about one million nephrons, which are the structural and functional units responsible for blood filtration. The kidneys also help in maintaining the acid-base balance, water balance, and blood pressure, and play a role in hormone secretion such as erythropoietin (for RBC production).

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.