Some people prefer interval training because
a. the intensity of the exercise doesn't increase, only the time does.
b. they've found that this type of training is best for beginners.
c. they can exercise at a steady, but slow, pace for longer periods of time.
d. it's performed with speed and may not cause boredom.

The answer is letter D. 

Interval training is usually performed by seasoned or well-trained individuals because of the high demand of speed and degrees of effort needed. Due to the nature of alternating exercise activities within the program, it is preferred by some individuals because it is fast and does not cause boredom. Beginners should not engage in this type of training because of its explosive nature. Through rigorous and a systematic training, beginners can eventually transition to this type of exercise provided that they have been previously cleared because they have already perfected and mastered the form of particular exercises.  

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