Correct Answer: C

Rationale: The correct answer is the large intestine. The large intestine is responsible for absorbing water and forming feces from undigested food. The stomach's main function is to break down food with acid and enzymes. The small intestine is where most of the nutrients are absorbed into the bloodstream. The esophagus is the tube that connects the mouth to the stomach and does not play a role in absorbing water or forming feces.

Correct Answer: C

Rationale: The correct answer is IgG. IgG is the only antibody that can cross the placenta, providing passive immunity to the developing fetus. IgA, IgD, and IgM do not cross the placenta. IgA is mainly found in mucosal secretions, IgD is involved in the activation of B cells, and IgM is the first antibody produced in response to an infection, but it does not cross the placental barrier.

Correct Answer: B

Rationale: The correct answer is B: 2. During aerobic respiration, each molecule of FADH2 produces 2 ATP molecules. FADH2 enters the electron transport chain and contributes to the generation of ATP. Choice A (1), Choice C (3), and Choice D (4) are incorrect because FADH2 specifically yields 2 ATP molecules per molecule in the process of aerobic respiration.

Correct Answer: B

Rationale: The correct answer is Iron. Iron is essential for the formation of hemoglobin, the protein in red blood cells that binds and transports oxygen throughout the body. Calcium (Choice A), Potassium (Choice C), and Sodium (Choice D) are not directly involved in the formation of hemoglobin and its oxygen-carrying function.

Correct Answer: D

Rationale: The correct answer is the cardiovascular system (Option D). The cardiovascular system is responsible for transporting oxygenated blood from the lungs, where oxygen is absorbed, to various tissues and organs in the body, including the spleen. The immune system (Option A) is responsible for defending the body against infections and diseases. The endocrine system (Option B) regulates hormones and their effects on the body. The respiratory system (Option C) is responsible for the exchange of gases, such as oxygen and carbon dioxide, in the lungs.