Which situation is an example of exponential decay?

A. The amount of a radioactive element is decreasing at a rate of 35% per year.
B. The weekly earnings of a part-time job that pays $8 per hour is increased by a weekly bonus of $25.
C. The amount deducted for taxes decreases an annual salary by about 22%.
D. The number of ants in an ant farm is increasing at a rate of 108% per week.