Which of the following are examples of exponential decay?

The population of a Florida is increasing by 43% each year.
The pesticide DDT has a half-life of 15 years.
March Madness has 64 teams in the bracket. Each round, half the teams are eliminated.
After an antibiotic is added to a culture of bacteria, the number of bacteria is reduced by half every three hours.
A tree frog population doubles every three weeks. btw this for algebra 1 they didnt have a subject for it