Correct Answer: A

Rationale: The corrected question and answer are accurate. The correct answer is A: Nucleus. The nucleus is known as the control center of the cell as it contains the cell's genetic material (DN
A) and directs all cell activities including growth, metabolism, and reproduction. The other organelles listed in the choices, while important for various cellular functions, do not serve as the central control center for overall cell activity.

Correct Answer: A

Rationale: The correct answer is A: Ribs. The ribs are part of the skeletal system, providing structure and protection to the thoracic cavity. The trachea, diaphragm, and alveoli are all essential components of the respiratory system. The trachea carries air to and from the lungs, the diaphragm aids in breathing by contracting and relaxing to change the volume of the thoracic cavity, and the alveoli are tiny air sacs where gas exchange occurs in the lungs.
Therefore, ribs do not play a direct role in the respiratory process and are not considered a part of the respiratory system.

Correct Answer: A

Rationale: The number of protons in an oxygen atom is a constant. This value is determined by the atomic number of the element and does not change regardless of the isotope of oxygen. The number of protons defines the element and remains consistent under normal conditions.

Choices B, C, and D are not constants. The temperature at which iron ore will melt can vary depending on factors like pressure, the human population size is constantly changing, and the time the sun rises each day varies with the seasons and location.

Correct Answer: B

Rationale: The correct answer is B because a water molecule consists of two hydrogen atoms and one oxygen atom, which is a fundamental chemical property of water. The pH of water is not a property of the water molecule itself but rather a measure of its acidity or alkalinity.
Choice C is incorrect as it describes a different molecule, not water.
Choice D provides an incorrect chemical formula for water, which is H2O.

Correct Answer: A

Rationale: The correct answer is A: Kidneys. The kidneys are part of the urinary system, responsible for filtering waste and maintaining fluid balance in the body. In the circulatory system, the heart pumps blood, blood carries oxygen and nutrients, and blood vessels transport blood throughout the body.

Choices B, C, and D are all components of the circulatory system, making the kidneys the correct choice as it belongs to the urinary system.