Help asap 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.

glamis thou art, and cawdor; and shalt be what thou art promis'd; yet do i fear thy nature; it is too full o' the milk of human kindness to catch the nearest way: thou wouldst be great; art not without ambition; but without the illness should attend it. what thou wouldst highly, that wouldst thou holily; wouldst not play false, and yet wouldst wrongly win: thou'dst have, great glamis, that which cries, "thus thou must do, if thou have it: and that which rather thou dost fear to do than wishest should be undone." hie thee hither, that i may pour my spirits in thine ear; and chastise with the valor of my tongue all that impedes thee from the golden round, which fate and metaphysical aid doth seem to have thee crown'd withal.  

he lacks ambition.  

he is hesitant.  

he lacks courage.  

he has moral scruples.

the maximum speed limit is 55 mph on a two-lane undivided highway and for vehicles towing trailers.

a. "to practice" is the best phrase to complete the sentence.

explanation:

during the summer, she likes to practice her soccer skills so that she is ready to play in the fall.

(note that : all the other options "the practicing of", "practice for" and "to do a practice on" are inappropriate as they make the sentence grammatically incorrect.)