A segment A of wire stretched tightly between two posts a distance $L$ apart vibrates in its fundamental mode with frequency $f.$ A segment B of an identical wire is stretched with the same tension, but between two different posts. You observe that the frequency of the third harmonic of wire B is the same as the fundamental frequency of wire A $.$

Part A

The length of wire B must be

$L.$

$3L.$

$9L.$