A String, Fixed at Both Ends, Vibrates at a Frequency

Multiple Choice

A string, fixed at both ends, vibrates at a frequency of 12 Hz with a standing transverse wave pattern containing 3 loops. What frequency is needed if the standing wave pattern is to contain 4 loops?

A) 48 Hz
B) 16 Hz
C) 36 Hz
D) 18 Hz
E) 12 Hz