Which of the following describes exponential decay?
a) The amount at a given time can be calculated by adding an amount to the previous amount.
b) The amount at a given time can be calculated by subtracting an amount from the previous amount.
c) The amount at a given time can be calculated by multiplying the previous amount by a value greater than 1.
d) The amount at a given time can be calculated by multiplying the previous amount by a value less than 1.