Correct Answer: C

Rationale: The correct answer is C: Pulmonary artery. The pulmonary artery carries deoxygenated blood from the heart to the lungs for oxygenation. This blood is considered the least oxygenated in the body as it has just returned from circulating through the body's tissues and is in need of oxygen replenishment. Choice A, the aorta, is incorrect as it carries oxygenated blood from the heart to the rest of the body. Choice B, the vena cava, carries deoxygenated blood from the body back to the heart. Choice D, the femoral vein, also carries deoxygenated blood back to the heart from the lower extremities.

Correct Answer: B

Rationale: The correct answer is B. Flies laying eggs in the food when it is left exposed to the air is a common occurrence and a logical explanation for the presence of fly larvae in spoiled food. Choice A is incorrect as food cannot evolve into fly larvae; fly larvae come from eggs laid by flies. Choice C is incorrect as fly larvae do not spontaneously generate from spoiled food but rather develop from eggs laid by flies. Choice D is incorrect as the presence of fly larvae in spoiled food is a real phenomenon, not merely imagined by people.

Correct Answer: A

Rationale: The number of protons in an oxygen atom is a constant. This value is determined by the atomic number of the element and does not change regardless of the isotope of oxygen. The number of protons defines the element and remains consistent under normal conditions. Choices B, C, and D are not constants. The temperature at which iron ore will melt can vary depending on factors like pressure, the human population size is constantly changing, and the time the sun rises each day varies with the seasons and location.

Correct Answer: B

Rationale: The correct answer is 'B: Recessive.' In genetics, eye color is determined by multiple genes, with green eyes often being associated with a recessive allele. When an individual carries two copies of the recessive allele for green eyes, they will express this eye color. Dominant alleles are expressed over recessive alleles. Homozygous refers to having two identical alleles for a particular gene, and heterozygous refers to having two different alleles. In this context, the allele for green eyes is recessive, meaning it is expressed only when an individual carries two copies of this specific allele. Therefore, 'Recessive' is the most appropriate term to describe the allele for green eyes, making it the correct answer.

Correct Answer: A

Rationale: The correct answer is A: Kidneys. The kidneys are part of the urinary system, responsible for filtering waste and maintaining fluid balance in the body. In the circulatory system, the heart pumps blood, blood carries oxygen and nutrients, and blood vessels transport blood throughout the body. Choices B, C, and D are all components of the circulatory system, making the kidneys the correct choice as it belongs to the urinary system.