Final answer:
The appropriate input (domain) for the function c(x) = 20x represents the numbers of calories burned per minute of exercise is x ≥ 0.
Step-by-step explanation:
The appropriate input (domain) for the function c(x) = 20x represents the numbers of calories burned per minute of exercise is x ≥ 0. In this case, x represents the number of minutes exercised, and since exercise cannot have a negative duration, the domain is restricted to non-negative real numbers.