Correct Answer: A

Rationale: The correct answer is the Thymus. T-cells mature and develop their receptors in the thymus gland, making it a vital organ for the immune system. The thymus provides the necessary environment for T-cells to differentiate and acquire their specific receptors and functions, which are essential for their role in the adaptive immune response. The spleen, lymph nodes, and bone marrow are all important components of the immune system, but they do not primarily serve as sites for T-cell maturation and T-cell receptor production.

Correct Answer: A

Rationale: The correct answer is the Thymus. T-cells mature and develop their receptors in the thymus gland, making it a vital organ for the immune system. The thymus provides the necessary environment for T-cells to differentiate and acquire their specific receptors and functions, which are essential for their role in the adaptive immune response. The spleen, lymph nodes, and bone marrow are all important components of the immune system, but they do not primarily serve as sites for T-cell maturation and T-cell receptor production.

Correct Answer: A

Rationale: The corrected answer is A: 'To exchange carbon dioxide for oxygen.' The alveoli in the lungs are responsible for facilitating gas exchange, where oxygen from inhaled air diffuses into the bloodstream, and carbon dioxide from the bloodstream diffuses into the alveoli to be exhaled. This process is vital for respiration and maintaining proper oxygen levels in the body. Choices B, C, and D are incorrect. Choice B is incorrect because the alveoli are not involved in transporting air to the lungs; they are primarily responsible for gas exchange. Choice C is incorrect because the alveoli do not filter blood; their main function is gas exchange. Choice D is incorrect because alveoli do not produce mucus; mucus is produced by other specialized cells in the respiratory system.

Correct Answer: A

Rationale: The endoplasmic reticulum is the correct answer. It is the cytoplasmic organelle that contains fluid-filled tubules responsible for carrying and transporting substances within the cell. The endoplasmic reticulum plays a crucial role in protein synthesis, lipid metabolism, and detoxification processes. The Golgi apparatus, although important in modifying, sorting, and packaging proteins for secretion or for use within the cell, does not primarily contain fluid-filled tubules for carrying and transporting substances. Mitochondria are responsible for generating ATP through cellular respiration, not for carrying and transporting substances. The nucleus contains the cell's genetic material and is involved in regulating cellular activities but does not contain fluid-filled tubules for transportation.

Correct Answer: A

Rationale: An atom's charge is determined by the difference between the number of protons and electrons. Since the atom has 17 protons and 19 electrons, the net charge is positive. The excess of two electrons compared to protons results in a positive charge. Choice B (Negative) is incorrect because the atom has more protons than electrons, leading to a positive charge. Choice C (Neutral) is incorrect because the atom has an imbalance between protons and electrons, resulting in a charge. Choice D (Cannot be determined) is incorrect as the charge can be determined based on the given information.