The answer your looking for is B that is what i think
Finding out if an exercise is safe needs to determine the fitness level of the exercise and how much it takes to achieve the result of the exercise. The person performing the exercise needs to know their endurance level and if they are able to perform the exercise without injury or illness.
A. Heart
(when you see "cardio",think heart)
Answer:
b I am pretty sure but now I'm second-guessing myself you also need sleep and sunlight and vitamins but I think you need food more. SORRY!