SAT Math Practice Question

The first year Eleanor organized a fund-raising event, she invited 30 people. For each of the next 5 years, she invited double the number of people she had invited the previous year. If $f(n)$ is the number of people invited to the fund-raiser $n$ years after Eleanor began organizing the event, which of the following statements best describes the function $f$ ?

1. A.The function $f$ is a decreasing linear function.
2. B.The function $f$ is an increasing linear function.
3. C.The function $f$ is a decreasing exponential function.
4. D.The function $f$ is an increasing exponential function.