A、With increasing blood. B、With more muscles. C、With vigorous trial. D、With routine exercise. D题目问的是心脏怎样才能变强大。由定位句可知，常规的日常训练可以使心

So what makes a world champion so strong? What qualities does he need?
    Muscles are made of thousands of stringy fiber 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.
    Aside from strength, two other ingredients go into making an athlete: fitness and endurance. 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 expand and contracts to take in blood and squeeze it out. Like any other muscles, the heart enlarges and gets stronger with routine exercise.
    Q25: Why is a world champion body builder so strong?
    Q26: What does an athlete need according to the passage?
    Q27: How does the heart enlarge and get stronger?