Which of the following are examples of exponential decay?

Exponential decay is the decrease in quantity N according to the law given by:
N(t)=Ne^(-λt)
for a parameter, t and a decay constant λ, N is the initial value. This process is common in areas such as radioactive decay, cooling in a draft, e.t.c. 
The example of decay from the list given includes:
-A pesticide DDT has a half-life of 15 years.
-After an antibiotic is added to culture bacteria, the number of bacteria is reduced by half every three hours.
-March madness has 64 teams in the bracket. Each round half the team is eliminated.