One of the Harmonics of a String Fixed at Both

Multiple Choice

One of the harmonics of a string fixed at both ends has a frequency of 52.2 Hz and the next higher harmonic has a frequency of 60.9 Hz. What is the fundamental frequency of the string?

A) 4.35 Hz
B) 8.7 Hz
C) 30.4 Hz
D) 17.4 Hz
E) 26.1 Hz