A、Because the fibers of his muscles are very thick. B、Because he has more muscles. C、Because his muscles are made of more string

A world champion body builder has no more muscles than does a 90-pound weakling. So what makes him so strong? What other qualities does he need?     
    Muscles are made of thousands of stringy fibers—a number that is fixed during childhood—which contract when doing work. Strength does not depend on the number of fibers but on the function of their thickness and how many of them contract simultaneously.
    Exercise actually damages the muscles. During the recovery stage, the muscle fibers increase in size. Exercise also trains more muscle fibers to work at one time. If a muscle is weak or untrained, for example, only about 10% of its fibers will contract, whereas up to 90% of the fibers in a weight lifter’s bulky biceps will contract.     
    Aside from strength, two other ingredients go into making an athlete: fitness and endurance. Fitness is related to the condition of the heart. During exercise, there is an increase in the amount of blood returning to the heart from the muscles. A typical volume for a runner at rest is about 5 quarts a minute, compared with 30 quarts during a vigorous trial. This greater volume means more work for the heart—a muscular balloon that expands and contracts to take in blood and squeeze it out. Like any other muscles, the heart enlarges and gets stronger with routine exercise.
    Endurance, or the length of time muscles can work, depends in part on how much fuel—in this case sugar—the muscles can store. A muscle that is continually exercised until it is exhausted of sugar tends to store more when it refuels at the next meal. And more sugar can translate into greater endurance the next time the muscle is put to the test.  
    Questions 12 to 15 are based on the passage you have just heard.
    12.Why is a world champion body builder so strong?
    13.What does an athlete need according to the passage?   
    14.How does the heart enlarge and get stronger?   
    15.What does endurance partly depend on?