The intensity, I, of a sound wave is defined as its power output per unit area and is measured in watts per square meter. A sound wave's intensity can be calculated using I=870pi^2a^2f^2, where a stands for the wave's amplitude in meters, and f stands for its frequency of vibration in hertz. If a particular sound wave has an intensity of 0.05 watts per square meter and an amplitude of 3 x 10^-5m, which of the following is most nearly its frequency? a)8.77 hertz b)60 hertz c)6,500 hertz d)4 x 10^7 hertz