Correct Answer: C

Rationale: The correct answer is Calcium (Choice C). Calcium is essential for bone health and is stored in bones by the skeletal system. Bones act as a reservoir for calcium, playing a critical role in maintaining bone strength and density. Iron, sodium, and magnesium are important for various bodily functions but are not primarily stored in bones for bone health. Iron is crucial for oxygen transport, sodium helps regulate fluid balance, and magnesium is involved in over 300 enzymatic reactions in the body.

Correct Answer: C

Rationale: The correct answer is C - 'Balanced equation'. A balanced equation is a representation of a chemical reaction that shows the reactants, products, and their stoichiometric coefficients. It ensures that the number of atoms of each element is equal on both sides of the equation, following the law of conservation of mass. Choice A, 'Hypothesis', is incorrect as it refers to a proposed explanation based on limited evidence as a starting point for further investigation. Choice B, 'Chemical formula', is incorrect as it represents the composition of a compound using symbols and subscripts. Choice D, 'Reaction mechanism', is incorrect as it describes the step-by-step process by which a chemical reaction occurs, not just the overall representation of the reaction itself.

Correct Answer: B

Rationale: During the S phase of interphase, DNA replication takes place. DNA polymerase is the enzyme responsible for synthesizing new DNA strands by adding nucleotides in a complementary manner to the template strand. It plays a pivotal role in accurately replicating the entire genome. While helicase unwinds the double-stranded DNA for replication, topoisomerase relieves the tension in the DNA strands, and ligase joins the Okazaki fragments on the lagging strand. However, DNA polymerase directly participates in the synthesis of new DNA strands during replication, making it the correct answer.

Correct Answer: A

Rationale: The correct answer is Growth hormone (GH). GH, produced by the pituitary gland, is responsible for stimulating the growth and development of bones, muscles, and organs. It plays a critical role in regulating growth during childhood and adolescence by promoting cell division, protein synthesis, and tissue growth. Thyroid-stimulating hormone (TSH) functions to stimulate the thyroid gland to produce thyroid hormones, while prolactin is involved in milk production. Follicle-stimulating hormone (FSH) is primarily responsible for reproductive functions like egg development in females and sperm production in males.

Correct Answer: D

Rationale: The Sinoatrial (SA) node is responsible for generating the electrical impulses that regulate the heartbeat. It is often referred to as the heart's natural pacemaker because it initiates the electrical signals that coordinate the heart's contractions. The other options listed (Aorta, Pulmonary veins, Coronary arteries) do not play a direct role in generating the electrical impulses for the heartbeat. The Aorta is a large artery that carries oxygen-rich blood away from the heart, the Pulmonary veins carry oxygen-rich blood from the lungs to the heart, and the Coronary arteries supply blood to the heart muscle itself, but none of them are involved in generating the electrical impulses that regulate the heartbeat.