Correct Answer: B

Rationale: Weight is the property of matter that refers to the force of gravity acting on an object. Mass is the amount of matter in an object, not affected by gravity. Density is the mass per unit volume of a substance, not directly related to gravity. Volume is the amount of space that an object occupies, not a measure of gravity's force.

Correct Answer: C

Rationale: The second harmonic (first overtone) is twice the frequency of the fundamental frequency. Therefore, the frequency of the second harmonic is 440 Hz * 2 = 880 Hz. The second harmonic has a frequency that is one octave higher than the fundamental frequency, representing the first overtone of the vibrating string. Choice A (220 Hz) is incorrect as it represents half the fundamental frequency and is the second harmonic, not the first overtone. Choice B (440 Hz) is the fundamental frequency itself. Choice D (1760 Hz) is the frequency of the fourth harmonic, not the second harmonic.

Correct Answer: D

Rationale: The correct answer is D, the Mitral valve. The mitral valve, also known as the bicuspid valve, is situated between the left atrium and the left ventricle. Its primary function is to prevent the backflow of blood from the left ventricle into the left atrium during ventricular contraction. The other choices are incorrect because: A) The aortic valve prevents backflow from the aorta into the left ventricle. B) The pulmonary valve prevents backflow from the pulmonary artery into the right ventricle. C) The tricuspid valve prevents backflow from the right ventricle into the right atrium.

Correct Answer: C

Rationale: Cortisol is the correct answer. It is the hormone produced by the adrenal glands that helps regulate the body's response to stress. Cortisol plays a crucial role in increasing blood pressure and suppressing the immune system during stressful situations. Aldosterone, another hormone from the adrenal glands, primarily regulates salt and water balance in the body, not the stress response. Epinephrine, also known as adrenaline, is produced by the adrenal glands in response to stress, but its effects are more immediate and short-lived compared to cortisol. Insulin, produced by the pancreas, regulates blood sugar levels and is not directly involved in the body's response to stress.

Correct Answer: A

Rationale: The balanced chemical equation shows that 4 moles of aluminum react with 3 moles of oxygen to produce 2 moles of aluminum oxide. Therefore, the mole ratio of aluminum to oxygen is 4:3. Choice B (2:3) is incorrect because it does not reflect the correct ratio based on the balanced equation. Choice C (3:2) and Choice D (3:4) are also incorrect as they do not represent the correct mole ratio of aluminum to oxygen according to the balanced chemical equation.