Exponential growth is when the rate of change of the value of any mathematical function[the change per unit of time] is proportional to the function's present value, resulting in its value at any time being an exponential function of time or becomes a function in which the time value is the exponent.
Exponential growth in a function is when the value will increase relative to its current value. Tripling is a dependent variable, and relative to the current value. Therefore, the correct answer is A.
