A string of length 0.4 m and mass 10–2 kg is tightly clamped at its ends. The tension in the string is 1.6 N. Identical wave pulses are produced at one end at equal intervals of time Δt. The minimum value of Δt which allows constructive interference between successive pulses is
(a) 0.05 s
(b) 0.10 s
(c) 0.20 s
(d) 0.40 s