Wave 1 Has Intensity $9.0 I_{0}$ While Wave 2 Has Intensity $4.0 I_{0}$

Wave 1 has intensity $9.0 I_{0}$ while wave 2 has intensity $4.0 I_{0}$ . What is their ratio of amplitudes, $A_{1} / A_{2}$ ?

A) 20
B) 1.3
C) 2.3
D) 1.5
E) 5.1