Arturo needs to write an explanatory text on how the human circulatory system works. Which of the following paragraphs would be the best introduction for his topic?

A. The human circulatory system causes blood to circulate throughout the body. It is an amazing system.
B. Of all the systems in the human body, the circulatory system may be the most complex. To understand it completely requires many years of study.
C. The heart is the main component of the human circulatory system. It beats from 60 to 100 times per minute, about 100,000 times per day, more than 30 million times per year, and about 2.5 billion times in a 70-year lifetime.
D. The human circulatory system is responsible for sending blood throughout the body, which provides it with oxygen and nutrients. The heart, the arteries, and the veins are just three of the parts that make up this amazing system.