To get a full destructive interference between two waves travelling in the same direction, a time delay of t = 3 sec is created between

To get a full destructive interference between two waves travelling in the same direction, a time delay of Δt = 3 sec is created between the two waves. The frequency of each individual wave is: r
f = 1/5 Hz r
f = 1/3 Hzr
f = 1/4 Hz r
f = 1/6 Hzr