Athletes warm up to ensure their joints are flexible. flexible joints are
a. important so you can run faster or reach farther.
b. important to prevent athletic injury.
c. only a concern for cross-training athletes.
d. naturally occurring as you move further into your fitness program.

The answer is letter B. 

Flexible joints are important to prevent athletic injury. Any good training program involves flexibility exercises because this ensures that joints and its surrounding tissues are lax and could perform at its peak during training and competition. Warm-up flexibility exercises are called active stretches. This is where an individual moves specific joints while simultaneously moving from one place to another. They are done before a training program begins while, passive stretches are done after a work-out. Passive stretches are used to remove lactic acid build-up in muscles and also reduces the chances of having stiff joints the day after a training session.

Ci want to say is right