Find a Formula for Estimating How Long Money Takes to Increase

Find a formula for estimating how long money takes to increase by a factor of 9 at R percent annual interest compounded continuously.

A) $\frac{100 \ln 18}{R}$
B) $\frac{100 \ln R}{9}$
C) $\frac{100+\ln 18}{R}$
D) $\frac{100 \ln 9}{R}$