A long glass tube is held vertically in water. A tuning fork is struck and held over the tube. Strong resonances are observed at two successive lengths 0.50 m and 0.84 m above the surface of water. If velocity of sound is 340 m s–1, then the frequency of the tuning fork is
(a) 128 Hz
(b) 256 Hz
(c) 384 Hz
(d) 500 Hz