Final answer:
It takes around B. 30 seconds for a red blood cell to circle the whole body.
Step-by-step explanation:
According to the given information, it takes blood about 20 seconds to make one complete transit through both the systemic and pulmonary circulations. Since the question asks for the time it takes for a red blood cell to circle the whole body, we can assume that red blood cells travel at a similar speed as the blood itself.
Therefore, based on the given information, it can be concluded that it takes around 30 seconds for a red blood cell to circle the whole body. The correct answer is B. 30 seconds.