decay every two years over a period of t years:
Fourth option: f(t)=(0.75)^(t/2)
decay 2 times a year over a period of t years:
Second option: f(t)=(0.9306)^(2t)
Sixth option: f(t)=(0.8660)^(2t)
decay once per year over a period of t years:
First option: f(t)=(0.9306)^t
Third option: f(t)=(0.8660)^t
