SAT math practice question: Exponential functions

 
Try the following SAT math practice question (Calculator Permitted) that tests your understanding of properties of exponential functions.

Question(Calculator Permitted): If $g$ is an exponential function and the value of $g(x)$ triples each time $x$ is increased by $9$, which of the following could define $g$?

1. $\quad g(x)=9^{x+18}$
2. $\quad g(x)=9^{x}+18$
3. $\quad g(x)=9^{\frac{x}{18}}$
4. $\quad g(x)=9^{18x}$