The term duration refers to
a. how strenuous a workout is.
b. how much sweat is lost during a workout.
c. how many times per week a person exercises.
d. how long a workout or exercise lasts.

The question is asking us to define the term "duration" when used for exercise - and in fact it is d. how long a work out or exercise lasts. For exmple, a duration of an exercise could be 90 minutes and the fruquency, that is how often you exercise, could be twice a week (this would be c, so if the question was asking for frequency, we would answer C)

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