A Stretched String of Length L,fixed at Both Ends,is Vibrating

Multiple Choice

A stretched string of length L,fixed at both ends,is vibrating in its third harmonic.How far from the end of the string can the blade of a screwdriver be placed against the string without disturbing the amplitude of the vibration?

A) L/6
B) L/4
C) L/5
D) L/2
E) None of these is correct.