Nokea
Under quiet conditions, blood returning to the heart retains about ________ of its oxygen content.
Correct Answer: C
Rationale: The correct answer is C (75 percent) because under quiet conditions, the venous blood returning to the heart still contains a significant amount of oxygen due to the efficient extraction of oxygen by tissues. This oxygen extraction process typically leaves around 25 percent of oxygen in the venous blood, which is then returned to the heart. Choices A (25 percent), B (50 percent), and D (90 percent) are incorrect because they do not accurately reflect the amount of oxygen retained in the blood under quiet conditions. Option A (25 percent) is the amount typically extracted by tissues, so it cannot be the correct answer. Option B (50 percent) is too high, as more oxygen is extracted by tissues. Option D (90 percent) is also too high, as it overestimates the amount of oxygen retained in venous blood.