The amount of a radioactive material changes with time. the table below shows the amount of radioactive material f(t) left after time t:

t(hours) 0 1 2
f(t) 100 50 25

which exponential function best represents the relationship between f(t) and t?
a. f(t) = 0.5(100)
b. f(t) = 100(0.25)
c. f(t) = 0.25(50)
d. f(t) = 100(0.5)