Maria believes that the daffodils in a field grow exponentially according to this model. Which statement explains why this is an exponential model?
A. The number of daffodils increases rapidly.
B. The number of daffodils is always a multiple of 2.
C. The number of daffodils is changing faster than the number of years.
D. The number of daffodils is always 1.5 times greater than the number in the previous year.