Correct Answer: C

Rationale: The given electron configuration matches that of Argon, an element found in Group 18 of the periodic table. This element is in the third period, as indicated by the highest energy level (n=3) where electrons are present. Therefore, the correct answer is Group 18, Period 3. Choice A (Group 16, Period 3) corresponds to sulfur, not the given electron configuration. Choice B (Group 14, Period 3) corresponds to silicon, not the given electron configuration. Choice D (Group 17, Period 2) corresponds to chlorine, which is in Period 3 but not in Group 18, making it incorrect for the given electron configuration.

Correct Answer: A

Rationale: The balanced chemical equation indicates that 2 moles of sodium react with 2 moles of water to yield 2 moles of sodium hydroxide and 1 mole of hydrogen gas. When 3 moles of sodium react with only 2 moles of water, sodium becomes the limiting reactant as it is present in excess compared to the available water molecules. This situation arises because not all sodium atoms can fully react with the limited amount of water, resulting in sodium being the limiting reactant in this specific case. Choice B (H2O), choice C (NaOH), and choice D (H2) are incorrect as they are not the limiting reactant in the given reaction scenario.

Correct Answer: A

Rationale: When a wave passes from one medium to another with the same speed, its frequency remains unchanged. Frequency is a characteristic of the source of the wave and does not depend on the medium through which the wave is traveling. Wavelength and speed of the wave can change when passing from one medium to another, but frequency remains constant. This is because the frequency of a wave is determined by the source that produces it, and as long as the speed remains constant, the frequency will not be altered. Amplitude, on the other hand, can change based on factors like energy loss or gain, but it is not a property that remains constant when a wave moves between different mediums with the same speed. Speed, although important for the wave's propagation, is not the property that remains unchanged when the wave transitions between mediums with the same speed. Therefore, the correct answer is frequency.

Correct Answer: D

Rationale: Frequent smoking can weaken the immune system by damaging immune cells and impairing their function. Smoking increases the risk of respiratory infections and other illnesses, negatively impacting immune health. Therefore, frequent smoking is a lifestyle practice that can weaken the immune system. Choices A, B, and C are all beneficial practices that support a healthy immune system. Getting enough sleep, eating a balanced diet rich in fruits and vegetables, and managing stress levels are important for maintaining optimal immune function and overall well-being.

Correct Answer: C

Rationale: Endotoxins are toxic substances that are part of the outer membrane of the cell wall of gram-negative bacteria. These toxins are released when the bacteria are destroyed or during bacterial growth. Endotoxins can cause a variety of harmful effects in the host, including fever, inflammation, and septic shock. Viruses do not produce endotoxins; they are typically associated with producing viral proteins that can be toxic to host cells. Gram-positive bacteria produce exotoxins, which are different from endotoxins found in gram-negative bacteria. Parasites, though capable of causing diseases, are not typically associated with the production of endotoxins.