Correct Answer: D

Rationale: The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction. In the given reaction, 2 moles of hydrogen combine with 1 mole of oxygen to form 2 moles of water. The total mass of the reactants (hydrogen and oxygen) is equal to the total mass of the products (water), meaning there is no decrease or increase in matter. The total mass of the system remains constant, demonstrating the conservation of mass. Choices A, B, and C are incorrect because the conservation of mass does not specifically relate to electrons, individual elements (like hydrogen), or the formation of new molecules; instead, it focuses on the overall mass of the system before and after the reaction.

Correct Answer: B

Rationale: The correct answer is B. The seven components of the digestive system are the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum. The mouth begins the process of digestion by chewing and mixing food with saliva. The pharynx serves as a passageway for food and air. The esophagus transports food from the mouth to the stomach. The stomach aids in digestion and acts as a temporary storage site. The small intestine is where most of the digestion and absorption of nutrients occur. The large intestine absorbs water and salts, forming feces. The rectum stores feces until they are eliminated through the anus. The other choices are incorrect as they do not list all seven essential components of the digestive system.

Correct Answer: D

Rationale: The correct answer is 'Inferior.' 'Inferior' refers to a position located towards the feet or lower part of the body. This is the opposite of 'superior,' which indicates a position towards the head or upper part of the body. 'Anterior' refers to the front of the body, 'posterior' refers to the back of the body, making them incorrect choices in this context.

Correct Answer: C

Rationale: In this scenario, where a child complains of heavy breathing even when relaxing and has no history of respiratory problems, the most likely issue is hyperventilation. Hyperventilation is an abnormal breathing pattern where a person breathes in excess of the body's requirements. It can be triggered by stress, anxiety, panic attacks, or lower-than-normal carbon dioxide levels in the blood. Unlike asthma, a blood clot, or exercising too hard, hyperventilation fits the symptoms described and is more common in individuals without a history of respiratory problems.

Correct Answer: B

Rationale: The correct answer is B: Contraction of the heart. Systole is the phase of the cardiac cycle during which the heart muscle contracts, pumping blood out of the chambers. It is essential for maintaining circulation and delivering oxygen and nutrients to the body's tissues. Diastole (choice C) is the relaxation phase of the heart when the chambers fill with blood. Pulse rate (choice D) is the number of heartbeats per minute, not specifically related to the systole phase. Choice A, 'Relaxation of the heart,' is incorrect because systole refers to the contraction phase of the cardiac cycle, not relaxation.